Existing Protein Machines

These protein hormones and enzymes selectively stick to other molecules. An enzyme changes its target's structure, then moves on; a hormone affects its target's behavior only so long as both remain stuck together. Enzymes and hormones can be described in mechanical terms, but their behavior is more often described in chemical terms.

But other proteins serve basic mechanical functions. Some push and pull, some act as cords or struts, and parts of some molecules make excellent bearings. The machinery of muscle, for instance, has gangs of proteins that reach, grab a "rope" (also made of protein), pull it, then reach out again for a fresh grip; whenever you move, you use these machines. Amoebas and human cells move and change shape by using fibers and rods that act as molecular muscles and bones. A reversible, variable-speed motor drives bacteria through water by turning a corkscrew-shaped propeller. If a hobbyist could build tiny cars around such motors, several billions of billions would fit in a pocket, and 150-lane freeways could be built through your finest capillaries.

Simple molecular devices combine to form systems resembling industrial machines. In the 1950s engineers developed machine tools that cut metal under the control of a punched paper tape. A century and a half earlier, Joseph-Marie Jacquard had built a loom that wove complex patterns under the control of a chain of punched cards. Yet over three billion years before Jacquard, cells had developed the machinery of the ribosome. Ribosomes are proof that nanomachines built of protein and RNA can be programmed to build complex molecules.

Then consider viruses. One kind, the T4 phage, acts like a spring-loaded syringe and looks like something out of an industrial parts catalog. It can stick to a bacterium, punch a hole, and inject viral DNA (yes, even bacteria suffer infections). Like a conqueror seizing factories to build more tanks, this DNA then directs the cell's machines to build more viral DNA and syringes. Like all organisms, these viruses exist because they are fairly stable and are good at getting copies of themselves made.

Whether in cells or not, nanomachines obey the universal laws of nature. Ordinary chemical bonds hold their atoms together, and ordinary chemical reactions (guided by other nanomachines) assemble them. Protein molecules can even join to form machines without special help, driven only by thermal agitation and chemical forces. By mixing viral proteins (and the DNA they serve) in a test tube, molecular biologists have assembled working T4 viruses. This ability is surprising: imagine putting automotive parts in a large box, shaking it, and finding an assembled car when you look inside! Yet the T4 virus is but one of many self-assembling structures. Molecular biologists have taken the machinery of the ribosome apart into over fifty separate protein and RNA molecules, and then combined them in test tubes to form working ribosomes again.

To see how this happens, imagine different T4 protein chains floating around in water. Each kind folds up to form a lump with distinctive bumps and hollows, covered by distinctive patterns of oiliness, wetness, and electric charge. Picture them wandering and tumbling, jostled by the thermal vibrations of the surrounding water molecules. From time to time two bounce together, then bounce apart. Sometimes, though, two bounce together and fit, bumps in hollows, with sticky patches matching; they then pull together and stick. In this way protein adds to protein to make sections of the virus, and sections assemble to form the whole.

Protein engineers will not need nanoarms and nanohands to assemble complex nanomachines. Still, tiny manipulators will be useful and they will be built. Just as today's engineers build machinery as complex as player pianos and robot arms from ordinary motors, bearings, and moving parts, so tomorrow's biochemists will be able to use protein molecules as motors, bearings, and moving parts to build robot arms which will themselves be able to handle individual molecules.

Evolving Organisms

The history of life is the history of an arms race based on molecular machinery. Today, as this race approaches a new and swifter phase, we need to be sure we understand just how deeply rooted evolution is. In a time when the idea of biological evolution is often slighted in the schools and sometimes attacked, we should remember that the supporting evidence is as solid as rock and as common as cells.

In pages of stone, the Earth itself has recorded the history of life. On lake bottoms and seabed, shells, bones, and silt have piled, layer on layer. Sometimes a shifting current or a geological upheaval has washed layers away; otherwise they have simply deepened. Early layers, buried deep, have been crushed, baked, soaked in mineral waters, and turned to stone.

For centuries, geologists have studied rocks to read Earth's past. Long ago, they found seashells high in the crushed and crumpled rock of mountain ranges. By 1785 - seventy-four years before Darwin's detested book - James Hutton had concluded that seabed mud had been pressed to stone and raised skyward by forces not yet understood. What else could geologists think, unless nature itself had lied?

They saw that fossil bones and shells differed from layer to layer. They saw that shells in layers here matched shells in layers there, though the layers might lie deep beneath the land between. They named layers (A,B,C,D..., or Osagian, Meramecian, Lower Chesterian, Upper Chesterian . . .), and used characteristic fossils to trace rock layers. The churning of Earth's crust has nowhere left a complete sequence of layers exposed, yet geologists finding A,B,C,D,E in one place, C,D,E,F,G,H,I,J in another and J,K,L somewhere else could see that A preceded L. Petroleum geologists (even those who care nothing for evolution or its implications) still use such fossils to date rock layers and to trace layers from one drill site to another.

Scientists came to the obvious conclusion. Just as sea species today live in broad areas, so did species in years gone by. Just as layer piles on top of layer today, so did they then. Similar shells in similar layers mark sediments laid down in the same age. Shells change from layer to layer because species changed from age to age. This is what geologists found written in shells and bones on pages of stone.

The uppermost layers of rock contain bones of recent animals, deeper layers contain bones of animals now extinct. Still earlier layers show no trace of any modern species. Below mammal bones lie dinosaur bones; in older layers lie amphibian bones, then shells and fish bones, and then no bones or shells at all. The oldest fossil-bearing rocks bear the microscopic traces of single cells.

Radioactive dating shows these oldest traces to be several billion years old. Cells more complex than bacteria date to little more than one billion years ago. The history of worms, fish, amphibians, reptiles, and mammals spans hundreds of millions of years. Human-like bones date back several million years. The remains of civilizations date back several thousand.

In three billion years, life evolved from single cells able to soak up chemicals to collections of cells embodying minds able to soak up ideas. Within the last century, technology has evolved from the steam locomotive and electric light to the spaceship and the electronic computer - and computers are already being taught to read and write. With mind and technology, the rate of evolution has jumped a millionfold or more.

Molecular Replicators

Cells replicate. Their machines copy their DNA, which directs their ribosomal machinery to build other machines from simpler molecules. These machines and molecules are held in a fluid-filled bag. Its membrane lets in fuel molecules and parts for more nanomachines, DNA, membrane, and so forth; it lets out spent fuel and scrapped components. A cell replicates by copying the parts inside its membrane bag, sorting them into two clumps, and then pinching the bag in two. Artificial replicators could be built to work in a similar way, but using assemblers instead of ribosomes. In this way, we could build cell-like replicators that are not limited to molecular machinery made from the soft, moist folds of protein molecules.

But engineers seem more likely to develop other approaches to replication. Evolution had no easy way to alter the fundamental pattern of the cell, and this pattern has shortcomings. In synapses, for example, the cells of the brain signal their neighbors by emptying bladders of chemical molecules. The molecules then jostle around until they bind to sensor molecules on the neighboring cell, sometimes triggering a neural impulse. A chemical synapse makes a slow switch, and neural impulses move slower than sound. With assemblers, molecular engineers will build entire computers smaller than a synapse and a millionfold faster.

Mutation and selection could no more make a synapse into a mechanical nanocomputer than a breeder could make a horse into a car. Nonetheless, engineers have built cars, and will also learn to build computers faster than brains, and replicators more capable than existing cells.

Some of these replicators will not resemble cells at all, but will instead resemble factories shrunk to cellular size. They will contain nanomachines mounted on a molecular framework and conveyor belts to move parts from machine to machine. Outside, they will have a set of assembler arms for building replicas of themselves, an atom or a section at a time.

How fast these replicators can replicate will depend on their assembly speed and their size. Imagine an advanced assembler that contains a million atoms: it can have as many as ten thousand moving parts, each containing an average of one hundred atoms - enough parts to make up a rather complex machine. In fact, the assembler itself looks like a box supporting a stubby robot arm a hundred atoms long. The box and arm contain devices that move the arm from position to position, and others that change the molecular tools at its tip.

Behind the box sits a device that reads a tape and provides mechanical signals that trigger arm motions and tool changes. In front of the arm sits an unfinished structure. Conveyors bring molecules to the assembler system. Some supply energy to motors that drive the tape reader and arm, and others supply groups of atoms for assembly. Atom by atom (or group by group), the arm moves pieces into place as directed by the tape; chemical reactions bond them to the structure on contact.

These assemblers will work fast. A fast enzyme, such as carbonic anhydrase or ketosteroid isomerase, can process almost a million molecules per second, even without conveyors and power-driven mechanisms to slap a new molecule into place as soon as an old one is released. It might seem too much to expect an assembler to grab a molecule, move it, and jam it into place in a mere millionth of a second. But small appendages can move to and fro very swiftly. A human arm can flap up and down several times per second, fingers can tap more rapidly, a fly can wave its wings fast enough to buzz, and a mosquito makes a maddening whine. Insects can wave their wings at about a thousand times the frequency of a human arm because an insect's wing is about a thousand times shorter.

An assembler arm will be about fifty million times shorter than a human arm, and so (as it turns out) it will be able to move back and forth about fifty million times more rapidly. For an assembler arm to move a mere million times per second would be like a human arm moving about once per minute: sluggish. So it seems a very reasonable goal.

The speed of replication will depend also on the total size of the system to be built. Assemblers will not replicate by themselves; they will need materials and energy, and instructions on how to use them. Ordinary chemicals can supply materials and energy, but nanomachinery must be available to process them. Bumpy polymer molecules can code information like a punched paper tape, but a reader must be available to translate the patterns of bumps into patterns of arm motion. Together, these parts form the essentials of a replicator: the tape supplies instructions for assembling a copy of the assembler, of the reader, of the other nanomachines, and of the tape itself.

A reasonable design for this sort of replicator will likely include several assembler arms and several more arms to hold and move workpieces. Each of these arms will add another million atoms or so. The other parts - tape readers, chemical processors, and so forth-may also be as complicated as assemblers. Finally, a flexible replicator system will probably include a simple computer; following the mechanical approach that I mentioned in Chapter 1, this will add roughly 100 million atoms. Altogether, these parts will total less than 150 million atoms. Assume instead a total of one billion, to leave a wide margin for error. Ignore the added capability of the additional assembler arms, leaving a still wider margin. Working at one million atoms per second, the system will still copy itself in one thousand seconds, or a bit over fifteen minutes - about the time a bacterium takes to replicate under good conditions.

Imagine such a replicator floating in a bottle of chemicals, making copies of itself. It builds one copy in one thousand seconds, thirty-six in ten hours. In a week, it stacks up enough copies to fill the volume of a human cell. In a century, it stacks up enough to make a respectable speck. If this were all that replicators could do, we could perhaps ignore them in safety.

Each copy, though, will build yet more copies. Thus the first replicator assembles a copy in one thousand seconds, the two replicators then build two more in the next thousand seconds, the four build another four, and the eight build another eight. At the end of ten hours, there are not thirty-six new replicators, but over 68 billion. In less than a day, they would weigh a ton; in less than two days, they would outweigh the Earth; in another four hours, they would exceed the mass of the Sun and all the planets combined - if the bottle of chemicals hadn't run dry long before.

Regular doubling means exponential growth. Replicators multiply exponentially unless restrained, as by lack of room or resources. Bacteria do it, and at about the same rate as the replicators just described. People replicate far more slowly, yet given time enough they, too, could overshoot any finite resource supply. Concern about population growth will never lose its importance. Concern about controlling rapid new replicators will soon become important indeed.

Evolution of Hominid Intelligence

How did hominid intelligence evolve? This question may never have a fully satisfying answer. It is unlikely that tool use led to bipedality, since the fossil record shows bipedality preceded larger brains and the earliest tools. Bipedaliy was more likely a response to the change of East Africa's climate from forest to savanna, where brachiating is less important than seeing over high grasses. Decreasing sexual dimorphism suggests more-monogamous pair-bonding, and supplemental male provisioning of nuclear families may have co-developed with bipedality. Concealed female ovulation and continual female sexual receptivity both probably contributed to increased male-male cooperation and tighter pair bonding. Such changing social patterns probably increased the selective pressure for intelligence, as did the availability of the hands for tool use following bipedality. The parallel increase in tool use and the size of the metabolically expensive brain was associated with a more omnivorous diet that included scavenging and hunting of meat.

How did Homo sapiens acquire language? This question, too, will likely never have a fully satisfying answer, as the fossil record tells even less about the development of language than it does about the development of intelligence. Just as sociality was crucial to the evolution of intelligence in animals, it probably also created selective pressure for the development of language skills. A variety of particular factors and stages have been proposed.

* Gestural and vocal imitation and miming
* Object and action reference by association through pointing or miming, perhaps motivated by the cooperative or imitative use of tools
* Counting as aiding a transition from iconic to symbolic representation
* Complex neural systems for perception or planned motor control as being adaptable to speech organ control and even syntax production
* Imperatives and interrogatives as earliest sentences
* Planning of future actions and understanding of others' intent as enabling more sophisticated syntactic structures

The result has been neural systems in the human brain that are highly specialized for language.

1.2.1.1. Philosophy / Epistemology / Philosophy Of Mind / Essence of Mind
A mind is any volitional conscious faculty for perception and cognition.

Cognition

Cognition is the process of learning, reasoning, and knowing. Learning is the processing of experience into an increase in knowledge or behavioral effectiveness. Reasoning is the process of making and evaluating valid inferences.

Perception

Perception is the process of organizing sensation into experience. Sensation is the process of external influence on a monitoring or control system. Experience is any relatively unified and coherent interpretation of related contemporaneous sensations.

Consciousness

Consciousness is awareness of self and environment. Awareness is the direct and central availability of information in a monitoring or control system.

Volition

Volition is the power or act of making decisions about an agent's own actions. A decision is the causing by a system of events which were not physically determined from outside the system but rather were at least somewhat contingent on the internals of the system, and which were not predictable except perhaps by modeling the internals of the system.

Free will is either of the doctrines that human choices are a) determined internally rather than externally (volitional free will) or b) not pre-determined at all (indeterminate free will). Determinism is incompatible with indeterminate free will, but is compatible with volitional free will if agents have internal state that influences (and thus helps determines) their actions. Volitional free will is also compatible with forms of indeterminism in which the acausality is not so rampant as to undermine agent self-influence. Indeterminate free will requires indeterminism, but degenerates into uncaused chance if acausality confounds not only prediction of effect but also attribution of cause.

Since most effects seem caused rather than uncaused, and since the complexity of minds makes them hard to predict, minds appear to have at least weak free will. Weak free will is sufficient for assigning ethical responsibility to decision-making systems even in the face of complete determinism.

Do minds have strong free will, or can their decisions in principle be inferred from sufficient knowledge of prior circumstances?

Anti-materialists posit an immaterial soul or will that is free from both deterministic causality and random acausality. This notion violates the law of the excluded middle. Either the immaterial will is subject to (perhaps probabilistic but nonetheless causal) causes, or it is not. The same is true of material minds. The actions of an immaterial will could be said to be caused by its own internal causal processes, but the same can be said of material minds.
1.2.1.2. Philosophy / Epistemology / Philosophy Of Mind / Accidence of Mind
Non-essential but perhaps inevitable aspects of mind include subjectivity, intentionality, and affect.

Subjectivity

Objectivity is independence from a point of view or perspective that is inherently private. Subjectivity is dependency on a point of view or perspective that is inherently private. Subjective experience is the private phenomenal aspect of experience, the vivid feeling of what an experience is "like".

Subjective experience consists of complex associations among perceptions, and necessarily occurs in systems having such associations. If a subjective experience is not "like" anything (i.e. not associated with any other perceptions), it is not a subjective experience at all.

Physicalism is the thesis that all facts can be described in physical (and thus non-subjective) terms. Some humans have what they call a "natural belief that collections of cells do not generate minds" [McGinn 1999] and that therefore physicalism must be false.

Such a belief seems only as "natural" as the belief that collections of atoms do not generate life, and just as unjustified. The operation of e.g. the human brain does not mysteriously causeconsciousness, but rather it simplyconstitutes consciousness.

Qualia are ineffable intrinsic subjective qualities of perception, such as the redness of red, beyond the functional or dispositional properties of perception. Qualia are taken by opponents of physicalism to be a mysterious phenomenon that physicalism cannot explain.

However, qualia do not exist, because the functional and dispositional properties of perception can, in fact, explain the subjective qualities of perception. The functional role of certain sorts of perceptions in a conscious system necessarily and understandably entails that the system will report qualia. Thus there are no ineffable intrinsic subjective qualities of perception beyond its functional qualities.

The Knowledge Argument is an argument made by Frank Jackson in 1982 purporting to show that physicalism is false because knowledge of all the relevant physical facts does not include, for certain experiences such as the redness of red, knowledge of what it is like to have them before they are had. Jackson hypothesizes in the distant future a brilliant neuroscientist Mary spending her whole life in a colorless room learning all the physical facts about seeing the color red. Jackson claims that only when Mary sees something red can she learns the new fact of what redness is like, and that therefore physicalism is false.

Jackson's argument fails because it ignores the difference between memorizing an algorithm and executing it. The experience of the redness of red consists in the operation of a complex set of functional components for processing information. While we can conceive of Mary having serial access to arbitrarily many memorized facts about such components, we cannot conceive of her having a large enough working memory or a fast enough mind to "manually" perform the operations "in her head" in order to recreate the experience of redness. Similarly, Mary could memorize the sequence of pixels in a monochrome bitmap and yet still not be able to mentally visualize what the bitmap will look like -- even if it is an image of a favorite drawing which she had already memorized in arbitrary detail.

A zombie is a hypothetical creature that is stipulated to lack subjective experience but is behaviorally and physically indistinguishable from a human. The conceivability or logical possibility of zombies is taken by opponents of physicalism to show that physicalism is false.

It seems impossible to conceive of a creature that lacks subjective experience but nevertheless exhibits all the self-reporting behaviors of humans that help us to ascribe subjective experience to them. Therefore, zombies are inconceivable and do not show physicalism to be false.

Intentionality

Intentionality is aboutness -- the property of being about, directed at, or suited for.

A system has intentionality by virtue of its potential and actual causal relations with the world.

The Chinese Room is a thought experiment devised by John Searle in 1980 to show that there cannot be intentionality or understanding in a formal symbol manipulation system such as a room in which a speaker of English manually executes an algorithm allowing the room to pass the Turing Test in Chinese. Searle claims that intentionality "is a biological phenomenon, and it is as likely to be as causally dependent on the specific biochemistry of its origins as lactation [or] photosynthesis". Searle charges that functionalism is a form of dualism because it says mind is in principle independent of the specific biochemistry of the brain.

The human in the Chinese Room does not understand Chinese, but the human running the algorithm implements a system that does indeed understand Chinese. The system has intentionality by virtue of the causal relations that allow it to correctly answer questions posed to it in Chinese. Intentionality is a formal or informational property, whereas lactation and photosynthesis involve chemistry and energy. Simulated thinking can indeed produce understanding, just as simulated musical composition can indeed produce a sonata. If a functional explanation of mind is "dualistic", then so is a functional explanation of long division or carburetion.

Affect

Affect is a general and often undirected negative or positive attitude, beyond overall sensory or cognitive state, that influences motive and colors perception. Is affect indeed an inevitable property of any volitional system with complex motives?
1.2.1.3. Philosophy / Epistemology / Philosophy Of Mind / Relations of Mind

Mind and Object

Concepts are abstractions induced by minds from instances. Concepts are the products of
the not-fully-understood facility by which a mind induces general properties from instances, and are themselves the not-fully-understood facility by which a mind recognizes those general properties in other similar instances.Ideas are concepts. Universals are kinds or categories of terms that are related according to shared properties. Human theories about universals are of three general kinds:

* Realism is the thesis that universals are essences that have existence independent of any instances.
* Conceptualism is the thesis that universals exist only as mental concepts.
* Nominalism is the thesis that universals are merely names given to groups of similar instances.

Universals do not exist independently of the instances that instantiate them and the minds that conceptualize them.

Mind and Minds

The Other Minds Problem is the problem of ascertaining whether external realty and other minds actually exist or merely appear to exist. Solipsism is the thesis that external reality and other minds do not actually exist. Solipsism incorrectly concludes not-X simply because X cannot be known with absolute certainty, and thus ignores the preferred conclusion of probably-X.

Mind and Identity

A mind is identical with its closest close-enough continuous-enough continuer. Processes that preserve mental (and thus personal) identity include:

* Deciding
* Incremental or continuous learning or forgetting
* Sleeping
* Locally continuous displacement through space, including e.g. teleportation via a portal
* Incremental transformation, e.g. through neuron-by-neuron replacement with computer chips

By contrast, the following processes do not preserve identity, usually because of being not continuous or not continuous-enough:

* Sudden irreversible complete amnesia
* Discontinuous teleportation, e.g. through disassembly and reassembly
* Discontinuous transformation, e.g. through mind uploading or restoration
* Reincarnation
* Duplication
* Simulation

It is perhaps logically possible for a single mind to fission continuously into more than one, or for more than one to continuously fuse into one, with identity being preserved in both cases. Chaotic or quantum effects probably make such fission or fusion physically impossible, since they might make it impossible to precisely synchronize the functioning of the duplicate components during the transition.

Mind and Spacetime

As noted by Dennett, the subjective sense of here -- the observer's spatial location -- is fixed by the content of mental events, and not by their spatial location. The subjective sense of now -- the observer's temporal location -- is similarly fixed by the content of mental events, and not by their temporal location.

Materialism implies that consciousness is distributed over space and time in a material substrate of mind such as the human brain. Thus there is no moment in time or point in space at which a thought enters consciousness. Asking when precisely did a material mind become conscious of an event is like asking when precisely did the British Empire learn of the signing of the treaty that ended the War of 1812. (The Battle of New Orleans was fought two weeks after the treaty was signed, by soldiers that had not yet heard of the signing.)

Mind and Artifact

Functionalism implies that, in principle, an artificial mind is possible, and that therefore a machine could think.

TheTuring Test is an assay for intelligence in which an interrogator using teletyped queries attempts to distinguish between a certified intelligence and a candidate intelligence. A rigorous interrogator can pose lines of questioning that can only be answered by use of the perceptive inductions that are the essence of intelligence. Not every intelligence could pass such a rigorous Turing Test, but everything that passes such a Turing Test is an intelligence.

Roger Penrose argues that the human mind is not computable because, given a formalization of one's mind and the Godel sentence for one's mind, a human mind allegedly could recognize the sentence as true whereas the formalized computation could not. Penrose errs in assuming one could know a formalization of one's mind and correctly believe in its consistency. Godel's Theorem merely shows that any formalizable reasoning faculty could not correctly believe in its own consistency.

Mind and Supermind

These are some of the levels of information-processing ability:

* Sentience is the capacity for sensation.
* Cognition is the process of learning, reasoning, and knowing.
* Consciousness is awareness of self and environment.
* Intelligence is the ability to make, test, and apply inductions about perceptions of self and world.
* Automentation is the ability of a mind to engineer all of its internal and external information storage and processing.

Automentation can be superior to regular intelligence in efficiency, flexibility, speed, capacity, bandwidth, and network associativity, but not in cognition. There are no forms of reasoning or kinds of knowledge that are in principle inaccessible to regular intelligence.

Mind and Limits

There are several ways in which minds are limited in theory and in practice.

* Logical Limits

* The Principle of Non-contradiction requires that no mind can correctly believe both a proposition and its negation. Indeed, all of the conclusions of logic are binding on all possible minds, as logic is in fact the study of valid inference. Computational Limits

If functionalism implies (as seems likely) that the logical limits on computability apply also to all physically possible minds, then several implications follow.
o Godel's Incompleteness Theorem would then imply that no mind with a formalizable reasoning system can be both consistent and complete.
o Neither the Decision Problem for the predicate calculus nor the (equivalent) Halting Problemcould be solved by any physically possible mind.
o Certain computational problems (such as sorting a list of N elements) could not be solved by any physically possible mind in less than an amount of time that is a function (such as log N) of the size of the problem.
* Epistemological Limits

It seems likely that no mind could ever achieve
o apodictic certainty about synthetic knowledge; or
o a proof of God's existence.
* Physical Limits
o Bremermann's Limit is the maximum processing speed (2´1047 bits per second per gram) of a self-contained material system. Bremermann's Limit derives from the Heisenberg's Uncertainty Principle and Einstein's principle of mass-energy equivalency. The finite age and mass of the universe combine with Bremermann's Limit to constrain the amount of thinking that any material mind can have done.
o The Bekenstein Bound is the physical limit of information density.
o Heisenberg's Uncertainty Principle implies that no mind can completely know the momentum of a particle at a particular position in space, or the energy of a particle at a particular moment in time.
o The finiteness of the speed of light limits how big and nimble a material mind can be, as well as how far it can sense or influence circumstances.
o The laws of thermodynamics require that no material mind in a closed system can create energy, decrease entropy, or indefinitely sustain a given level of operation.
o No material mind can travel backwards in time. The ability to do so would eliminate the mind's computational limits and physical limits.
* Biological Limits

* Biological minds necessarily inherit a powerful drive to sustain self and kind. However, intelligence and volition can in principle allow a mind to overcome any such biological imperative. Psychological Limits

Human minds are subject to many psychological limits in the areas of memory, perception, attention, concentration, volition, and cognition.