Before anything, just want to say sorry for the long reply.
Firstly, no, I don’t think that’s per se the problem, IF, in the first place, it’s granted there’s such a thing as abstraction/conception.
For, as I’ve stated in my post which you’ve replied to, abstraction/conception, in general, can’t occur without the very principle of reason; again, if otherwise was possible, we could have examples of abstracts/concepts that violate the principle or condition of reason, &, therefore, are inherently contradictory, e.g., such as a square circle. Yet since we don’t & can’t, this suffices to show that reason isn’t an effect of or derived from abstracts/concepts, as they don’t & can’t condition its rules, but vice versa.
Think of it like this, anything that’s an effect of or derived from abstracts/concepts can be altered by them, such as a “pegasus” having wings made of carrots as opposed to feathers; yet whatever abstracts/concepts can’t alter, was never something which was an effect of or derived from abstracts/concepts, such as their inability to alter a square or a circle into a square circle because neither a square nor a circle was an effect of or derived from abstracts/concepts but sensations (which are independent of them); this latter reasoning, again, of what they’re incapable of altering not being an effect of or derived from them, is to be applied to reason itself, thus demonstrating its independence from them (as sensible objects, like a square or a circle, are).
Now what the problem really is, as I see it, is how the materialist can demonstrate the being of abstraction/conception by purely physical means (& not per se how reason arises from the former [so let’s not get ahead of ourselves, as this is something to be explained only after the fact of demonstrating how conception/abstraction arises from physicality]). For, in the context of the mind, induction is an actual thing, i.e., thinking certain things about the future (for example, causal connections [a-la Hume & Kant]), yet no physical thing, such as a set of neurons, can transcend its present state & actually refer to the future in order to make claims about it. So, in my view, the problem of the materialist per se isn’t explaining reason arising from abstracts/concepts, but, in the first place, the latter on a purely physical basis.
Now, moreover, I agree that reason can’t appear to us without causality, yet this doesn’t necessarily mean that reason is an appearance, but only that our initial knowledge of it occurs by means of a conditional application of it to some object.
“And to say reason is merely one of two necessary human conditions doesn’t shed any light on its fundamental origin.” — Sure, but my opposition to the claim that it arises from abstraction/conception isn’t so much about what its fundamental origin is, as what its fundamental origin IS NOT, i.e., conception/abstraction.