§ 51. Marks or Attributes.

Those properties by which we recognize any object, and assign it a place under some appropriate conception, are called its marks. If these are invariably found in the objects of a given sort, they are called essential; if only a portion of the class possesses them, they are accidental. The whole of the essential marks of a species make up its specific character, or its essence. Two marks which are in the very mode of expressing them opposed to each other, as wise and unwise, mortal and immortal, are called contradictory, because it is impossible to assign them to the same object without a contradiction in terms; and this is certain à priori, because the one is the mere negation of the other, so that their opposition does not depend on an examination into the nature of these marks. If they were represented as A and not-A, we should be as sure that they were diametrically opposed, as if A was a word of well-known meaning, instead of an arbitrary symbol. Marks which are opposed to each other, but not as a positive and negative, so that we know their contrariety à posteriori, from experience, as sweet and sour, hard and fluid, are termed repugnant marks. Those which may meet in the same object, as sweet and fluid, sour and hard, we may call compatible.

§ 52. Extension and Intension.

When we compare a vague and general conception with a narrower and more definite one, we find that the former contains far more objects in it than the latter. Comparing plant with geranium, for example, we see that plant includes ten thousand times more objects, since the oak, and fir, and lichen, and rose, and countless others, including geranium itself, are implied in it. This capacity of a conception we call its extension. The extension of plant is greater than that of geranium, because it includes more objects.*

But conceptions have another capacity. Whilst plant has more objects under it than geranium, it has fewer marks in it. I can describe the leaves, petals, stamina, and pistils of geranium; but of plant no such description is possible. I cannot say that every plant has a stem, for there are the lichens to contradict me; nor a flower, for ferns have none, and so on. I can say little more about plant, than that all plants have growth and vegetable life. The logical expression of this defect is, that its intension is very limited.

* Mr. Mill, Logic, 1. vii. 1., thinks it only "accidental" that "general names should be the names of classes. But his own language contradicts him; if they are general they belong to genera; it cannot be accidental that a class-name should be the name of a class.

The greater the extension, the less the intension; the more objects a conception embraces, the more slender the knowledge which it conveys of any of those objects; and vice versâ.*

With the help of the important distinction between extension and intension, or, as others express it, the sphere and matter of the conception, magnitudo et vis conceptûs, we can understand the meaning of the saying-that the subject of a judgment is in the predicate, and the predicate in the subject. "Man is an animal;" this conveys two notions, that man is contained in animal, as a species in a genus; and that whatever makes up our notion of animal-all the marks of animal-are contained in (vráρxε †) man. So they are mutually contained.

* The various modes of expressing the double capacity of conceptions are as follows:

† Aristotle (Anal. Pri. I. i., and many other places) adopts in preference this mode of putting the proposition. Instead of "Man is an animal," he has "Animal inheres in man."

§ 53. Determination.

The reverse of the abstractive process, that of descending from higher conceptions to lower, by resuming the marks laid aside, is called determination. Thus from the broad class of diseases, we determine or mark out the class of fevers, by the peculiar symptoms of heat, rapid pulse, &c., which are their marks ; and from fevers we descend further to intermittent fevers, by bringing in the fresh mark of time.

As abstraction augments the extension by diminishing the marks, so determination augments the intension by increasing them. Notions of individuals, and they only, are said to be fully determined, because to them there are no more marks to add. The use of the word determination in its logical sense is already sanctioned by our older writers.

§ 54. The three powers of a Conception.

That all simple cognitions have three powers or a threefold value, in that they consist of marks, and include objects, and are summed up in names, has been stated already. To these three functions as many processes correspond; Division of a Conception enumerates all the objects or classes that are included under it, and so deals with the extent of the notion;