Few modern readers, I fear, will read this explanation with the same zest which St Thomas evidently felt in writing it. Kantian idealism on the one hand, and physical science on the other, have averted the modern mind -- is it for ever? -- from species intelligibilis and intentio intellecta, or verbum mentale. Accidents, scientifically considered, as colour, odour, shape, are not to us what they were to the mediaeval schoolman. We busy ourselves with the sensation of colour, the effect on retina and brain and inner consciousness, and further with the vibrations from without that are apt to set up such a sensation in a creature organised as man is. And at the back of colour we discern with the mind's eye, what the bodily eye is insensible to, a colourless, invisible molecular structure, and a complication of interacting forces all but infinite in multitude, all but infinitesimal in power. Whoever would rehabilitate Thomist philosophy to the requirements of modern science, has before him work for a lifetime, no old man's labour. One thing however I will say about the 'likeness' (similitudo) here said to obtain between the thing in itself and our impression or idea of the thing. There can be no question here of any such likeness as obtains between a portrait, or photograph, and the person who sits for it. What can be maintained on behalf of Realistic Dualism is this, that between the impression or idea in consciousness and the thing in itself there is a certain correlation or proportion, inasmuch as the thing in itself, striking our senses and thereby our understanding, is apt to induce in us certain sensations and consequent ideas. These aptitudes, or potentialities, relative to man, are the objective properties, or accidents, of the thing in itself as cognizable by man. This doctrine is simply an extension to all substance of a conclusion generally received in respect to those interesting substances whom we call our friends and acquaintances. We have impressions and ideas of them, gathered from their conversation and their dealings with us. We trust that our friends are at heart such as their conversation represents them. If they are not, they are false and deceitful, or at least unknowable and unlovable persons; and there is an end of friendship. But assuming that our fellow-men, or some of them, as things in themselves, answer to our impressions and ideas of them, what of horses and dogs, and the lower sentient creation generally? What again of plants, of minerals and gases? Are they not all so many potential energies, to some extent impressing us, but in great measure beyond us, and even when away from us still real? And in the ascending scale, what of angels and of God? These are interesting questions to all except the solipsist. Abandon solipsism, and any extreme form of idealism becomes impossible; nay, it may be found necessary to come to terms with Realistic Dualism. Does not monism spell solipsism?
I have translated similitudo 'likeness,' but the intelligent reader will take it to mean no more than 'proportion,' or 'correspondence,' of the impression or idea in the mind with the thing in itself. 'Things in themselves' are knowable in point of their aptitudes in our regard, aptitudes which remain potential, and do not drop to zero, when not exercised. If any one will venture on the fatal denial of potentiality, and assume that, as in God, so also in the creatures of God, nothing is but what is actualised, no logic can save him from the last excesses of pantheism.
Of God and His Creatures: 1.53