(3.) _Opinion_ finds in _a priori_ judgements no place whatever, for by them we either cognise something as quite certain or else cognise nothing at all. But if the given grounds of proof from which we start (as here from the purposes in the world) are empirical, then we cannot even with their aid form any opinion as to anything beyond the world of sense, nor can we concede to such venturesome judgements the smallest claim to probability. For probability is part of a certainty possible in a certain series of grounds (its grounds compare with the sufficient ground as parts with a whole), the insufficient ground of which must be susceptible of completion. But since, as determining grounds of one and the same judgement, they must be of the same kind, for otherwise they would not together constitute a whole (such as certainty is), one part of them cannot lie within the bounds of possible experience and another outside all possible experience. Consequently, since merely empirical grounds of proof lead to nothing supersensible, and since what is lacking in the series of them cannot in any way be completed, we do not approach in the least nearer in our attempt to attain by their means to the supersensible and to a cognition thereof. Thus in any judgement about the latter by means of arguments derived from experience, probability has no place. (4.) If an _hypothesis_ is to serve for the explanation of the possibility of a given phenomenon, at least its possibility must be completely certain.[143] It is sufficient that in an hypothesis I disclaim any cognition of actuality (which is claimed in an opinion given out as probable); more than this I cannot give up. The possibility of that which I place at the basis of my explanation, must at least be exposed to no doubt; otherwise there would be no end of empty chimeras. But to assume the possibility of a supersensible Being determined according to certain concepts would be a completely groundless supposition. For here none of the conditions requisite for cognition, as regards that in it which rests upon intuition, is given, and so the sole criterion of possibility remaining is the mere principle of Contradiction (which can only prove the possibility of the thought, not of the object thought).