A contradiction is involved in the supposition that an idea or universal can become the object either of sense or of the imagination. An idea is thus diametrically opposed to an image, although in ordinary, and even in philosophical language, the two terms are frequently confounded, and regarded as synonymous with each other.
26. I have hitherto spoken of necessity and universality as two main characteristics of our ideas. I have now to remark that ideas are essential to the unity of our cognitions. They are not merely indefinite possibilities which no given number of instances can exhaust, but they are principles by which the variety and multifariousness of our sensible impressions are reduced to unity and order. Resemblance, for example, is the great principle of arrangement and classification. We class things together under genera and species according to their resemblance. But resemblance does not come to us through the senses, or by the way of sensation; it is no sensible impression, it is a pure idea. When two trees are before us, we see the trees, but we do not see their resemblance. This is a thought, not an object of sense. Resemblance is a relation, and, as such, it cannot be seen, or touched, or apprehended by any of the senses. These apprehend only the things. Their relations of resemblance and difference are apprehended only by the intellect. If the mind had no idea of resemblance, and no idea of difference, if we
