As a consequence of my growing interest in epistemology, I recently read some articles about concepts. The authors were originating from different fields, such as philosophy of mind, language and knowledge. Some were psychologists.
It seems to be well known to everybody working on concepts that only a few (if any) uncontroversial claims can be made about them. However, the following claims seem to be quite widely accepted, at least by philosophers:
(1) Concepts are the building blocks of thought.
(2) We refer to concepts using words.
(3) Concepts are the basis by which we categorize the world.
(4) Communication is possible only to the extent that we share concepts.
Now what is puzzling me is how claims (2) to (4), taken together, can make genuine philosophical disagreements possible. Let me illustrate my concern with an example:
Given the concept that I name "science", I may start to wonder what exact set of practices fall under this concept (this is a formulation of the so-called demarcation problem). Let's say that, after reflection, I come to consider that psychology deserves to fall under the concept "science". Let's also suppose that Alice, a competent English speaker, asks herself the same question and come to believe that psychology is not a science. Then the extension of our respective concepts that we name "science" differ. But for our disagreement to be more than merely verbal (which I believe is true), our "science" concepts must be the same. That means that concepts are not individuated by their extension (this claim seems also to be relatively uncontroversial).
My question is therefore: if concepts are not individuated by their extension, on which basis should I attribute a concept possession to other speakers ? If Alice says that physics are not a science, should I consider that she possesses the concept I name "science" ?
I know that some philosophers have suggested to make a distinction between concepts and conceptions of concepts. But then, my question can be rephrased to: "How do I know that Alice and I are refering to two different conceptions of the same concept, and not two distinct concepts (which would be a case of verbal dispute) ?".
Thank you very much for your help.