This is more a statement than a question, and you've framed the question in a way that partly pre-judges it. The word "external" implies there is an "internal" to the brain/mind. That is a common view from the information processing metaphor for cognition, but it is just that -- a metaphor -- and there are other views than the information processing one. Obviously the world isn't purely a cognitive construct. It exists. Claiming that "our constructions of external reality are cognitive shifts from one brain level to another level" is a *very* big claim. Do you have sources for this, or evidence for it?

"It exists". These are two words of English language which do not exist outside the cognitive processes of my brain.There is no way that you can demonstrate empirically that "(the world ) it exists". That is the conclusive judgment of verifiability epistemic criterion of the most harshest of Western philosophical school-- the logical positivism.Simply put ,a "brain-less comprehension" of the world and its constituents is ad absurdum.(Mind is an obsolete concept because it does not exist,therefore,I am using brain.It is real).

Our concepts and theories about the external world(that which exists outside our brain and its cognitive processes)The two examples, which follow will explain what is meant by cognitive shifts.The Special Relativity of Einstein explains motion and gravity,entirely different from the Newton's classical mechanics.That changed whole theorizing about the structure of physical reality.Theory of Evolution was another cognitive shift from the classical Biblical creationism.There are endless examples scattered in the annals of history and philosophy of science.This also applies to other scientific domains such as Biology,Geology,Sociology,Economics.The work of Thomas Kuhn,Imre Lakatos,Paul Feyerabend,Karl Popper can further enlighten you in this connection.The change from locomotives to email is but a simple demonstration of cognitive shift.My great grandfather had no knowledge about a place called "Canada" but i do.Thanks to cognitive shifts.(Moreover I can cognitively construct the place called Canada).

I'm no logical positivist, but I still disagree. We speak the same language, so "it exists" has a status independent of my brain. If I write it on paper, other people can read it and know what I meant. Language is a synchronization through social consensus. And I don't even need to get to Descartes to say that.

Your basis, as I understand it, is a form of eliminative materialism. That is a position you can take, but it is not completely trivial to defend. In additional to the usual responses, yours is brain-centric. Just because brain is the basis for current mental phenomena, it does not follow that brain is necessary in principle for mental phenomena. And what about animal brains? So while I see what you mean a little more clearly, I can also more easily respond to it. For example, to understand my state when I write "I just shoveled a 30cm of snow" you cannot reduce this wholly to a brain state. The concept of a centimetre, for example, is based on social consensus. There is nothing in my brain that is sufficient to understand this sentence. Extreme reductionism -- which is what you are advocating -- needs a much better defence for its extreme claims.

I've read Kuhn et al with interest. But I don't buy their model of science. I prefer that of Larry Laudan, for example, and the picture in Barbara Von Eckhardt's excellent book on the philosophy of cognitive science. Also, Mike Mulkay's "Three Models of Scientific Development" is worth a read. Basically, radical shifts are not as common as people like to think -- far more commonly several perspectives co-exist in smaller communities, branching and merging, some growing, other fading. Personally, I'm afraid, I don't find eliminative materialism an especially persuasive direction for these reasons.

I am simply stating that our epistemic judgments are essentially cognitive.I can not come to know,for example that the external world exist just because a number of people socially consented in a particular language that , 'it exists'.(and what about those who do not understand this particular language,surely, 'it exists' does not exist for them). One can not say that Newton's third law of motion is a social consensus on the use of certain kind of mathematical language agreed upon by physicists.Similarly, Einstein's mass-energy equation is not a social agreement.Its a cognitive event in the first place followed by its confirmation through repeated experiments.It is a cognitive construct followed by repeated observations in different parts of the world.Similarly,Parkinson syndrome is a neurological disorder connected with genetic structure of human beings.It is not a synchronization of social consensus, rather the converse holds in this case.

Now animal brains.Take the example our our closest neighbors,chimps and monkeys,They can not solve a simple quadratic equation.They can not understand and solve tensor calculus involved in the solutions of General Theory of Relativity. They even can not perform a simple addition/subtraction function the way a human child does.I am just looking for some putative epistemological criterion for judging whether there is an external world or not.That i can not arrive at if and only if i presume that my epistemic criterion is a social consensus and nothing else. That i venture to state is supplied by the cognitive processes going on in our brains.

Similarly , "centimeter" was invented by a French ad later on put to social use by the scientific institutions.Your own statement that you shoveled 30 cm of snow clearly demonstrate that certain concepts connected with an external world condition were cognitively processed by your brain, which led you to the necessary action. Imagine for a moment that lost your entire 'memory' when you saw this white thing called snow, falling in front of main entrance of your home,will you be able to do what you claimed in your statement, "i just shoveled a 30 cm snow". I am sure not.

I am not advocating an extreme reductionism. I am just trying to unlock the riddle.Where a picture of the world is first processed/formed.? Ahmad

The external world exists, this is the first step, processing of "our" brain build a world other than that of origin (fisical world) but shared by all of us, as follows processing of precise laws. For example the visual pathways are the same in all men...

You're still taking the information processing model of cognition for granted. I'd look at Gibson's ecological perception as an interesting alternative - mainly because it will show that some of these assumptions are just that - assumptions. (I stand by claiming that language - not cognition - is rooted in social consensus, and that even though Parkinson's existed before we talked about it, our concept of it is a social consensus, and it is very likely to be cluster of related syndromes). Shovelling is an interesting example. Even without memory, the world has affordances that relate it directly to my body/mind. I can walk without memory. I can manipulate objects without memory. I probably could use my hands to clear snow without memory. A well-designed shovel will afford grasping in ways that afford its use. I obviously couldn't say that I cleared snow, but your confidence that I would be unable to do it, in principle, may be misplaced. If so, cognition has its roots in our evolutionary history (and Ruth Millikan would make the same claims about concepts, too, as might Mark Johnson) so finding a singularity-type point of origin may be a wild goose chase.

Shoveling without memory? Gibson paradigm will not stand up to that claim of yours.There is obvious gap between afffordance of "shoveling" with effcetivity required to do it. You will be unable to make sense of your environment.then how will you perform it?I am not taking information processing as a model of human knowledge.Human ability to create new knowledge in an unpredictable way is unique.While animal brains keep on repeating the same behavior performances within their narrow environment.The human cognitive ability can respond to unforeseen and unforeseeable patterns of environment.Otherwise we would have never been able to invent so much.