[Symmetry]

I think you underrate this line of thought. The existence of neurons is, of course, not justified in the philosophical sense - but its hard to argue that anything is justified in the philosophical sense. Reason might be a source of justification, but for that to be so you first have to assume that reason is justified. Senses might be a source of justification, but first you have to assume that knowledge derived from senses if justified. If you do not make either of these assumptions you have to retreat form justified to probabilistic or tentative knowledge, and in those terms the existence of neurons is a rather more reasonable assumption.

[somephilosopher]

It's not like philosophy is interested in some special notion of 'justified' - philosphy tends to be interested in our ordinary notion of justification.

We normally think that the belief that neurons exist is justified. Philosophy will ask what that amounts to. It may turn out that the belief is unjustified - but most philosophers will reject this. They will argue over different accounts of what that justification consists in.

[Symmetry]

Modern philosophy has indeed moved beyond this notion of justification, but for a long time it was the central project of Western philosophy. What were the rationalists, empiricists, Kantians, and positivists arguing about other than the justification of knowledge (in this sense).

EDIT: Maybe I'm going overboard here in attributing to philosophy in general what is only true of epistemology, my favorite branch of it. In that case, I should have been referring to "epidemiological justification".

[somephilosopher]

Weeeeelll, yes and no. But mostly no.

There has been a strand of 20 cent philosophy that has regarded justification as that which together with truth and belief yields knowledge - if you do that, you're individuating justification in terms of a particular epistemic role, and that so it's a theoretical notion. That might be what you're getting at.

But most philosophers nowadays use the notion of 'warrant' for that. So for them, the notion of justification doesn't have that particular theoretical role.

Generally, why be interested in a theoretical notion? The fundamental questions in Epistemology are questions like:

* What is knowledge? * What is justification? * Which beliefs are justified? * What do we know, and how do we know it? etc etc

Those are the starting points, and they are phrased in English. They use ordinary notions. Given that, no answer to them which first redefines the terms to yield distinct questions is going to suffice as an adequate answer.

Philosophers use a lot of technical jargon, and are interested in technical questions. But generally, those questions have pretty clear connections to our ordinary ways of thinking about the world.