Full Idea
A transcendental proof converts a possibility into a necessity: by saying under what conditions experience of objects is possible, transcendental proofs show those conditions to be necessary for us to the extent that we have any experience of objects.
Gist of Idea
Transcendental proofs derive necessities from possibilities (e.g. possibility of experiencing objects)
Source
Sebastian Gardner (Kant and the Critique of Pure Reason [1999], 02 'Transc')
Book Reference
Gardner,Sebastian: 'Kant and the Critique of Pure Reason' [Routledge 1999], p.45
A Reaction
They appear to be hypothetical necessities, rather than true metaphysical necessities. Gardner is discussing Kant, but seems to be generalising. Hypothetical necessities are easy: if it is flying, it is necessarily above the ground.