While space and time could be chosen as apriori forms in the subject, thinking cannot lead one to postulatethe existence of an orthoexistence. However, since this may bea mental conclusion rather than a mental choice, this conception assumes the value of a philosophical experiment. What this orthoexistencemay be, say, as an additional coordinate, is rather a speculativethought or, at best, a hypothesis. Given that no critique of experimentalreason has thus far been made, the philosophical experiment cannotbe subject to a critical examination. Nor can we use in this respectthe hosts of topical ideas on the relation between theory(which is pure, a priori with Kant) and experiment.
Although Kant refers to pure reason on thevery ground that it is interesting in relation to experiment,for the theoretical/experimental relation can alone lead to cognition,he advances critical views on formal logic. His reserves are legitimateespecially nowadays, when the procedures associated with formal logic can be so easily automated and so this logic does not appearto be a sufficiently creative and inventive resource of the humanmind.
With Kant, pure reason is man's ability todevise theoretical models and ever new languages. It is his abilityto sound into the objective reality. However, in defect of experience,the models have no truth - or cognition - value. This is no doubtso since all the schemas of pure reason are not cognition from the beginning. "To think an object and to cognize an object are by no means the same thing"7.Cognition is both conception and experiment. That is why Kantobserves that "All mathematical conceptions, therefore, arenot per se cognition". The experiment develops byintuition (a form of sensibility with Kant). In defectof intuition, the concept is just an idea - form against which nothing can be actually moulded in concreto. There existshowever pure intuition (based on a priori space and time)which does not rely on experiment, but this intuition is onlyconducive to the form of phenomena rather than to their content.
However, Kant also refutes a more empiricalderivation of cognitions, criticizing John Lock and David Hume.He is no doubt right for our understanding cannot be clear untilwe build the internal model.

Kant thought that the empirical and the theoretical(i.e. pure reason) only coexist as distinct moments; they do notmerge into one another, nor do they exist as symbiotic elements.
Reason appears to him to be indeed pure withrespect to experiment. Reason is assumed to work also in defectof sensibility, the feeling of pleasure and of pain are not cognitions.One can no doubt impose (or require) that reason should dispensewith sensibility, but in reality sensibility will inherently workin man. The ground of using pure reason, unaffected bysensuous feelings may itself contain a sensuous constituent. This is not however a serious matter. Sensibility may be reduced,or just minor with respect to pure reason. What really mattersis that by this clear-cut separation of the empirical and the pure (i.e. the a priori, the theoretical), he ruled outany experimental and, in a way, empirical introspection whichwe shall call experimental reason or philosophical experiment.This is in all evidence an ultimate consequence of the thing-in-itself,which precludes reason and internal experiment to develop concomitantly,to be inseparably fused onto the internal unity of man.
Kant recognizes the "internal sense"but he does not confuse it with pure reason. It rather looks likeanother kind of external sense, as if it would regard the ownsubject in its appearance. The internal sense relates, of course, to reason, but like the external sense, it "brings"us qua phenomenon, hence as we appear, rather than as weare as thing-in-itself.
A thorough reason-experiment fusion can onlybe obtained by building a material world more profound than existencein space and time. In the philosophical experiment, the subjectwas taken as it is without adding or subtracting anything. Thatis why it leads to results that imply orthoexistence and go beyondKant's thing-in-itself.
Once the break, i.e. the lack of fusion (forcoexistence is possible) between reason and empirical is judged in terms of the internal empirical, it makesa breach in a vulnerable part of Kant's theory, i.e. that of apperception.

To Kant8 there exist three original sources of any cognition: thesense (which gives the manifold), imagination (whichgives the synthesis of the manifold) and apperception,which is a more subtle phenomenon, related to the fundamentalphilosophical experiment of consciousness. All these faculties can be of both empirical and pure (in Kant's terms a priori) use.Let us now examine apperception, which is crucial in any philosophicalexamination of knowledge. In Kant's view, apperception is self-consciousness.It is originally given as an empirical apperception, whichis right. However, self consciousness gives birth to the judgementand the representation "I think" which must necessarilybe capable of accompanying all human representations. However,according to Kant, the unity of self-consciousness indicating"the possibility of a priori cognition arising from it" is, as a judgment, pure apperception. Later on Kant observes that"The first pure cognition of understanding, then, upon whichis founded all its other exercise, and which is at the same timeperfectly independent of all conditions of mere sensuousintuition, is the principle of the original synthetical unityof apperception"9.

The Limit of the Thing-in-Itself11