XLVII. If of our own proper genus, we know nothing with certainty, what must our knowledge be of others? The genus most immediately next to our own, is that of plants, and in this, notwithstanding our proximity, we can discern nothing but our ignorance; for we cannot venture even by conjectures, to point out the constitutive difference between them. This is not only invisible to the eyes of evidence, but impalpable to the essays of opinion. We commonly define the genus of a plant, by the term insensible living entity. But the word insensible, which we use to describe the difference, only signifies want of sensibility; and a positive entity, such as a plant is, cannot be ascertained by a negative. Besides this, as we observed before, it is somewhat doubtful, whether plants are, or are not sensible. We call them also _living vegetables_. But by this idea, we do not assign to the plant a different nature from the animal, for that this also is a living vegetable. If I am told, that the difference consists, in that the life of the animal is vegetative and sensitive, and that that of the plant is purely vegetative, I reply, that the adverb _purely_ in this place, signifies nothing but the want of sensitive life, which is meant to be expressed in the extreme, and a want, cannot be made use of as a descriptive term, to point out the constitutive difference between positive entities. Nor will it be of any use to answer, that the term want, means a want or defect in the mode of explanation, and does not apply to the thing signified; for till it can be ascertained what the thing signified is, we must remain totally in the dark; and it is also false, that this term _want_, is not applicable to the thing signified; for negative expressions are positive with respect to the thing signified, when they are used to deny any imperfection in the object they relate to; for the want of an imperfection, is the want of a want, it being certain, that all imperfection consists in the want of positive perfection; for which reasons these words, infinity, immensity, indivisibility, although negative with respect to the mode of expression, are positive with respect to the thing signified; but the term insensible, or insensibility, applied to a plant, signifies want of perfection, and thus is negative with respect to the thing signified.