The transcendental aesthetic, Kant's discovery, uses what we immediately experience as a subject (the subjective), to discover that which remains invariant by the properties of that immediate experience, necessarily existing in each human experience, and thus across all human experiences--this is known as the relativistic-objective, a good definition for a universal truth at least in the human experience (this movement from a subjective experience to the objective by universalization is the transcendental!). This is possible because our experience, by the nature of experience, is limited by our perception--thus being at the limit of what can be known; however this also means each subject has the same pure intuitions in order to have experience at all: causality, space, and time. The result is, one, communication is possible, but two, we can have universal knowledge of the 'world' because all experiences are derived from those four intuitions and thus will always be confined by them.

I shouldn't give you the answer, but rationality ends up being justified by the pure intuition of causality not because it's a special game but instead it happens to be universal in the inter-subjective as a pure intuition because it's a requirement for the 'subject' to exist at all.

I've taken some liberties to make this concise, but I really recommend to read at least The Prolegomena. You can also go through some of my previous comments to see others I've left on similar topics.