For Peirce, thoughts are not enclosed within our brains or individual minds – and neither are percepts. If they were, perception could not open up the cognitive bubble as it sometimes does.
When we first wake up to the fact that we are thinking beings and can exercise some control over our reasonings, we have to set out upon our intellectual travels from the home where we already find ourselves. Now, this home is the parish of percepts. It is not inside our skulls, either, but out in the open. It is the external world that we directly observe. What passes within we only know as it is mirrored in external objects. In a certain sense, there is such a thing as introspection; but it consists in an interpretation of phenomena presenting themselves as external percepts. We first see blue and red things. It is quite a discovery when we find the eye has anything to do with them, and a discovery still more recondite when we learn that there is an ego behind the eye, to which these qualities properly belong. Our logically initial data are percepts. Those percepts are undoubtedly purely psychical, altogether of the nature of thought. They involve three kinds of psychical elements, their qualities of feelings, their reaction against my will, and their generalizing or associating element. But all that we find out afterward. I see an inkstand on the table: that is a percept. Moving my head, I get a different percept of the inkstand. It coalesces with the other. What I call the inkstand is a generalized percept, a quasi-inference from percepts, perhaps I might say a composite-photograph of percepts. In this psychical product is involved an element of resistance to me, which I am obscurely conscious of from the first. Subsequently, when I accept the hypothesis of an inward subject for my thoughts, I yield to that consciousness of resistance and admit the inkstand to the standing of an external object. Still later, I may call this in question. But as soon as I do that, I find that the inkstand appears there in spite of me. If I turn away my eyes, other witnesses will tell me that it still remains. If we all leave the room and dismiss the matter from our thoughts, still a photographic camera would show the inkstand still there, with the same roundness, polish and transparency, and with the same opaque liquid within. Thus, or otherwise, I confirm myself in the opinion that its characters are what they are, and persist at every opportunity in revealing themselves, regardless of what you, or I, or any man, or generation of men, may think that they are. That conclusion to which I find myself driven, struggle against it as I may, I briefly express by saying that the inkstand is a real thing. Of course, in being real and external, it does not in the least cease to be a purely psychical product, a generalized percept, like everything of which I can take any sort of cognizance.
EP2:62
And of course, the form of this ‘purely psychical product’ is partially determined by the physical form of the perceptual process, which depends on the perceiver’s embodiment. The percepts of a color-blind person will not be the same as those of someone with normal color vision, although they will agree on the externality of the object they are perceiving, and will both attribute whatever color-qualities they see to that object, as neither of them has any control of those qualities. Yet through dialogue, they may become aware that their percepts differ, and thus become aware of aspects of perception internal to the nervous system.
These internal aspects of the perceptual process are themselves products of development and evolution, habits in the broad Peircean sense, varying somewhat from body to body. Sometimes those who become conscious of these habits as such can take control of them to some degree, even though one does not normally control one’s own developmental process, and its “schedule” is mainly determined by factors beyond anyone’s control. For instance, one who has no opportunity to learn language before puberty is unlikely to learn it later on in life, as the developmental “window” for taking on that set of habits has passed.
Another example is stereoscopic vision (the perception of depth resulting from the brain’s ‘fusing’ of the two different images received by the two eyes). People vary considerably in the degree of stereoscopic perception they have, and some do not develop it at all because they lack an eye or normal alignment of the two eyes as babies. Usually, if the defect in alignment is corrected later in life, it’s too late for the person to develop the habit of stereoscopic vision. But Oliver Sacks (2010, 111-143) describes the case of ‘Stereo Sue,’ who learned in middle age how to see in stereo depth, and had to work very hard at the eye exercises prescribed by her optometrist in order to maintain this ability even after she had learned how to do it.
The plasticity of the human brain allows for some limited conscious control even of perceptual processes, and although conceptual processes are much more malleable, there is no fixed boundary between them. Likewise there is no fixed boundary between the internal and external worlds; all phenomena involve some interaction or ‘coupling’ between them. Lack of control of psychical or mental phenomena is not an absolute criterion of external reality either. People who are subject to hallucinations may be fully aware that they are not external objects of perception, not real in that sense, and yet have no control of their appearance (Sacks 2012).