Since necessity is the same thing as following from given grounds, it must appear in a special way in the case of every form of the principle of sufficient reason, and also have its opposite in the possibility and impossibility which always arises only through the application of the abstract reflection of the reason to the object. Therefore the four kinds of necessity mentioned above stand opposed to as many kinds of impossibility, physical, logical, mathematical and practical. It may further be remarked that if one remains entirely within the province of abstract concepts, possibility is always connected with the more general, and necessity with the more limited concept; for example, “An animal _may_ be a bird, a fish, an amphibious creature, &c.” “A nightingale _must_ be a bird, a bird _must_ be an animal, an animal _must_ be an organism, an organism _must_ be a body.” This is because logical necessity, the expression of which is the syllogism, proceeds from the general to the particular, and never conversely. In the concrete world of nature (ideas of the first class), on the contrary, everything is really necessary through the law of causality; only added reflection can conceive it as also contingent, comparing it with that which is not its cause, and also as merely and purely actual, by disregarding all causal connection. Only in this class of ideas does the conception of the _actual_ properly occur, as is also shown by the derivation of the word from the conception of causality. In the third class of ideas, that of pure mathematical perception or intuition, if we confine ourselves strictly to it, there is only necessity. Possibility occurs here also only through relation to the concepts of reflection: for example, “A triangle _may_ be right-angled, obtuse-angled, or equiangular; its three angles _must_ be equal to two right-angles.” Thus here we only arrive at the possible through the transition from the perceptible to the abstract. After this exposition, which presupposes the recollection of what was said both in the essay on the principle of sufficient reason and in the first book of the present work, there will, it is hoped, be no further doubt as to the true and very heterogeneous source of those forms which the table of judgments lays before us, nor as to the inadmissibility and utter groundlessness of the assumption of twelve special functions of the understanding for the explanation of them. The latter point is also supported by a number of special circumstances very easily noted. Thus, for example, it requires great love of symmetry and much trust in a clue derived from it, to lead one to assume that an affirmative, a categorical, and an assertatory judgment are three such different things that they justify the assumption of an entirely special function of the understanding for each of them.