Kant, in various parts of his treatment of causality, refers to determinism or the principle of sufficient reason as an inescapable principle. In fact, in the Second Analogy we find the elements to reconstruct a purely phenomenal determinism as a logical and tautological truth. I endeavour in this article to gather these elements into an organic theory of phenomenal causality and then show, in the third section, with a specific argument which I call the "paradox of phenomenal observation", that this phenomenal determinism is the only rational approach to causality because any logico-reductivistic approach, such as the Humean one, would destroy the temporal order and so the very possibility to talk of a causal relation. I also believe that, all things said, Kant did not achieve a much greater comprehension of the problem than Hume did, in his theory of causality, for he did not free a phenomenal approach from the impasse of reductivism as his reflections on "simultaneous causation" and "vanishing quantities" indeed show, and this I will argue in Sect. 4 of this article.