[It is an] error ... to suppose that cognitive systems are located inside people's heads. Rather, cognitive systems are largely in the world. I no more carry my complete cognitive systems around with me as I walk from place to place than I carry the U.S. currency system about with me when I walk with a dime in my pocket.

Ruth Millikan, White Queen Psychology and other essays for Alice, MIT Press, 1995, p.170


According to the version of functionalism that I originally proposed, mental states can be defined in terms of Turing machine states and loadings of the memory (the paper tape of the Turing machine). ... computational models of the brain/mind will not suffice for cognitive psychology. We cannot individuate concepts and beliefs without reference to the environment. Meanings aren't "in the head".

H. Putnam, Representation and reality, MIT Press, 1988, p.73


Knowledge, say you, is only the perception of the agreement or disagreement of our own ideas: but who knows what those ideas may be? Is there anything so extravagant as the imaginations of men's brains? Where is the head that has no chimeras in it? Or if there be a sober and a wise man, what difference will there be, by your rules, between his knowledge and that of the most extravagant fancy in the world? They both have their ideas, and perceive their agreement and disagreement one with another. If there be any difference between them, the advantage will be on the warm-headed man's side, as having the more ideas, and the more lively. And so, by your rules, he will be the more knowing. If it be true, that all knowledge lies only in the perception of the agreement or disagreement of our own ideas, the visions of an enthusiast and the reasonings of a sober man will be equally certain. It is no matter how things are: so a man observe but the agreement of his own imaginations, and talk conformably, it is all truth, all certainty. Such castles in the air will be as strongholds of truth, as the demonstrations of Euclid. That an harpy is not a centaur is by this way as certain knowledge, and as much a truth, as that a square is not a circle.

But of what use is all this fine knowledge of men's own imaginations, to a man that inquires after the reality of things? It matters not what men's fancies are, it is the knowledge of things that is only to be prized: it is this alone gives a value to our reasonings, and preference to one man's knowledge over another's, that it is of things as they really are, and not of dreams and fancies.

To which I answer, That if our knowledge of our ideas terminate in them, and reach no further, where there is something further intended, our most serious thoughts will be of little more use than the reveries of a crazy brain; and the truths built thereon of no more weight than the discourses of a man who sees things clearly in a dream, and with great assurance utters them. But I hope, before I have done, to make it evident, that this way of certainty, by the knowledge of our own ideas, goes a little further than bare imagination: and I believe it will appear that all the certainty of general truths a man has lies in nothing else.

But what shall be the criterion of this agreement? It is evident the mind knows not things immediately, but only by the intervention of the ideas it has of them. Our knowledge, therefore is real only so far as there is a conformity between our ideas and the reality of things. But what shall be here the criterion? How shall the mind, when it perceives nothing but its own ideas, know that they agree with things themselves? This, though it seems not to want difficulty, yet, I think, there be two sorts of ideas that we may be assured agree with things.

As all simple ideas are really conformed to things. First, The first are simple ideas, which since the mind, as has been shown, can by no means make to itself, must necessarily be the product of things operating on the mind, in a natural way, and producing therein those perceptions which by the Wisdom and Will of our Maker they are ordained and adapted to. From whence it follows, that simple ideas are not fictions of our fancies, but the natural and regular productions of things without us, really operating upon us; and so carry with them all the conformity which is intended; or which our state requires: for they represent to us things under those appearances which they are fitted to produce in us: whereby we are enabled to distinguish the sorts of particular substances, to discern the states they are in, and so to take them for our necessities, and apply them to our uses. Thus the idea of whiteness, or bitterness, as it is in the mind, exactly answering that power which is in any body to produce it there, has all the real conformity it can or ought to have, with things without us. And this conformity between our simple ideas and the existence of things, is sufficient for real knowledge.

All complex ideas, except ideas of substances, are their own archetypes. Secondly, All our complex ideas, except those of substances, being archetypes of the mind's own making, not intended to be the copies of anything, nor referred to the existence of anything, as to their originals, cannot want any conformity necessary to real knowledge. For that which is not designed to represent anything but itself, can never be capable of a wrong representation, nor mislead us from the true apprehension of anything, by its dislikeness to it: and such, excepting those of substances, are all our complex ideas. Which, as I have shown in another place, are combinations of ideas, which the mind, by its free choice, puts together, without considering any connexion they have in nature. And hence it is, that in all these sorts the ideas themselves are considered as the archetypes, and things no otherwise regarded, but as they are conformable to them.

J. Locke, An essay concerning human understanding, 1690.

If John Locke were a present-day neurophysiologist, he would have been probably talking about neurons firing in response to the appearance of objects in front of the observer.

Locke's notion of representation by covariation (between an external event and an internal representation) has been especially popular with neuroscientists, for whom it has replaced the Aristotelian idea of representing a shape by having a small replica of it inside one's head. The computer vision people preferred, however, to stick with Aristotle.

It has been pointed out that the firing of a neuron tuned to a certain shape is not a reliable indicator of the presence of that shape in the field of view. The neuron tuned to a cat shape may respond at a fraction of its maximal firing rate to a dog, or an atypical cat. This shortcoming is remedied by adopting Shepard's idea of 2nd-order isomorphism.


The isomorphism should be sought --- not in the first-order relation between (a) an individual object, and (b) its corresponding internal representation --- but in the second-order relation between (a) the relations among alternative external objects, and (b) the relations among their corresponding internal representations. Thus, although the internal representation for a square need not itself be square, it should (whatever it is) at least have a closer functional relation to the internal representation for a rectangle than to that, say, for a green flash or the taste of a persimmon.

R. N. Shepard and S. Chipman, Second-order isomorphism of internal representations: Shapes of states, Cognitive Psychology 1:1-17, 1970, p.2.


I [...] suggest an alternative approach, in which the outside world is considered as a kind of external memory store, which can be accessed instantaneously by casting one's eyes (or one's attention) to some location. The feeling of the presence and extreme richness of the visual world is, under this view, a kind of illusion, created by the immediate availability of the information in this external store.

J. K. O'Regan, Solving the real mysteries of visual perception: The world as an outside memory, Canadian J. of Psychology 46:461-488, 1992.


Ordo et connexio idearum idem est ordo et connexio rerum.

[The order and the connection of ideas is the same as the order and the connection of things.]

B. Spinoza, Ethics II, 7.

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