Why are concepts without intuitions blind?

Question

I think at this point I understand all the transcendental arguments of CPR except this one - and probably this could considerably change my understanding of Kant as a whole.

Here is my confusion.

1. Intuitions are a priori.
2. Concepts are a priori.
3. This might be a bit contestable but Kant in various locations does state pure appearances exist (intuitions without concepts). But they serve no value for us since it makes no judgement. There cannot be a true/false value attached to it, since it's not a judgement.
4. However, we DO apply concepts without intuitions. This is the mistaken field of metaphysics as explicated in Transcendental Dialectic.

Now, if 1 and 2 are true, and Kant can grant 3 (however, this is not crucial to my argument), with what exact reasoning can he state that 4 is 'empty'. I can understand if he says they are 'useless'. But in the transcendental dialectic, his position is more than these metaphysical statements being useless - it is that they are incorrect/erroneous. His position is that it is a mistake to use concepts without intuitions.

For this, in my opinion, he will need to elucidate that concepts CANNOT exist without intuitions, i.e, their content is derived from intuitions. However, it cannot be 'derived' by definition, since they are a priori. On top of that we know they do exist independently in the field of metaphysics - Kant himself claims this.

So while, as a Nietzchean perhaps, I can grant Kant that concepts applied to things that cannot be present in intuition is probably useless (this is an ethical argument, not an epistemological one), I still cannot grant Kant an ontological reason for why I should not use concepts without intuitions in my discussions (which is what metaphysics did before him).

And yes, this argument is a part of Transcendental Deduction, but it's (a) quite unclear and (b) not satisfactory in my opinion because just because categories are a presupposition of experience which also involve intuition in no way mean applying them without intuitions is illegitimate.

EDIT:

I realized this question can be posed in a different way. (This is partly due to the fact the answer might just resort to the fact that synthetic a priori judgements require predicates that we can only get through intuitions).

1. Are metaphysical judgements synthetic a priori? If not, how are they possible. They are not analytical, of course.
2. If they are synthetic a priori, how can they be mistaken, given that they are a priori and the entire system of Kant depends universalizing that which is a priori.

Answer 1

Kant never (to my memory) uses the term a priori intuition. He uses the term formal intuition or pure intuition which is probably what you mean. The contrast is significant, however, as your question shows - disregarding it leads to some interpretative problems. Ascribing a priority to intuitions could suggest that they could acquire their unity independently of spontaneity of the Understanding (i.e. without involvement of pure concepts/the categories) but this isn't right - cf. the discussion of the function that gives the unity to an intuition at B131. Intuitions already have conceptual shape. This constitutes the thesis that intuitions are blind without concepts.

The Transcendental Dialectic, Kant asserts, consists of an indirect proof of his formal idealism, i.e. the thesis that thinking has no content of its own and has to depend on sensory intuition in order to refer to a real object (which is more than a empty logical possibility but actual object of empirical knowledge, cf. Preface to the second edition of the Critique where Kant discusses this in a footnote). This is the character of the human intellect that it has to depend on the intuition. But it of course can attempt to - and Kant says this is indeed necessary and natural (Preface to the first edition of the Critique, also the very beggining of the Transcendental Dialectic where he discusses the notion of transcendental illusion) to fall into this metaphysical obscurity - overcome the bounds of experience, as thought is independent of affection (being given an object in an intuition) at least in principle. The Dialectic which, Kant says, is an experiment of reason (again, Preface to the B edition) concludes that this way, without pure or empirical intuition, no knowledge can be achieved so the thesis of formal idealism, that thought has no content of its own, is reasserted and the distinction between appearances and things in themselves are posited. Why? To bring Reason into harmony with itself, i.e. to resolve the contradictions by undermining the faulty assumptions that give rise to them. If you don't want that, then Kant really cannot do anything to convince you, it seems.

You say that this constitutes a paradox, because concepts are a priori. Indeed, they are, but they're also formal. They represent a mere mannor ('rule') of combining/synthesis, Kant says, of a prior, externally given, sensible manifold of contents - and do not have any content of their own. Concepts, thus, can exist independently of intuitions while acquiring any content at all only in relation to intuition (the mannor in which this happens is elaborated by the Analytic of Principles). The pure a priori concepts in application to the pure intuition of time are called the schemata [of the categories] - and these already have sensible content. These are the scaffolding of empirical knowledge. But pure concepts alone aren't even that - they're mere plans with no reference to reality without external assistence.

To briefly answer your reformulation: Kant asks in the Introduction to the Critique of Pure Reason: How are a priori judgements of metaphysics possible? He answers: they aren't. So, you can say they're only in principle a priori because in reality no such thing as metaphysical knowledge exists.