124. The opinion of St. Thomas is not a mere conjecture: we may, in order to establish it in some manner upon fact, assign a reason which seems to us greatly to strengthen it, and which may be regarded as merely an extension of the one already given. A thing to be intelligible must have two qualities: immateriality, and the activity necessary to operate upon the intelligent being. This activity is indispensable, for in the act of intelligence, the intellect is in some sense passive. When the idea is present, the intellect cannot but know it: when it is wanting, it is impossible for the intellect to know it. The idea, therefore, enables the intellect to act; without it the intellect can do nothing. Consequently, if we admit that any being can serve as idea to the intellect, we must concede that being an activity to excite intellectual action; and so far we make it superior to the intellect excited. Thus we explain why our intellect, in this life at least, does not know itself by itself. Experience shows that its activity needs to be excited. Left to itself it is like one asleep; and this want of activity in our mind, in the absence of exciting influences, is one of the most constant of psychological facts. This is not, however, to say that we have no spontaneity, and that no action is possible without an external determining cause; but only that this same spontaneous development would not exist, if we had not previously been subjected to the influence of causes which brought out our activity. We may learn things not taught us; but we could learn nothing, if teaching had not presided over the first development of our mind. There are, it is true, many ideas in our mind, which are not sensations, and which cannot have emanated from them; but it is equally true that a man, deprived of all his senses, could not think, because his mind would want the exciting cause. 125. We have dwelt thus long upon the explanation of the problem of intelligibility, because we consider it of scarcely less importance than that of intelligence, although we do not find it treated in philosophical works as it merits. We will now reduce this doctrine to clear and simple propositions, so that the reader may form a more complete conception of it; and also, in order to deduce some consequences which have been only slightly indicated in our exposition: I. A thing must be immaterial in order to be immediately intelligible. II. Matter cannot be intelligible by itself. III. The relations of spirits to bodies, or the representation of the latter in the former, cannot be purely objective. IV. Some other class of relations must necessarily be admitted to explain the representative union of the world of intelligences with the corporeal world. V. Immediate objective representation supposes activity in the object. VI. The power of an object to represent itself to the eyes of an intelligence, supposes in it a faculty of acting on that intelligence.