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phantasms, as we have said above (Q. 84, A. 7). Thus it clearly appears that immaterial substances which do not fall under sense and imagination, cannot first and _per se_ be known by us, according to the mode of knowledge which experience proves us to have.
Nevertheless Averroes (Comment. De Anima iii) teaches that in this present life man can in the end arrive at the knowledge of separate substances by being coupled or united to some separate substance, which he calls the "active intellect," and which, being a separate substance itself, can naturally understand separate substances. Hence, when it is perfectly united to us so that by its means we are able to understand perfectly, we also shall be able to understand separate substances, as in the present life through the medium of the passive intellect united to us, we can understand material things. Now he said that the active intellect is united to us, thus. For since we understand by means of both the active intellect and intelligible objects, as, for instance, we understand conclusions by principles understood; it is clear that the active intellect must be compared to the objects understood, either as the principal agent is to the instrument, or as form to matter. For an action is ascribed to two principles in one of these two ways; to a principal agent and to an instrument, as cutting to the workman and the saw; to a form and its subject, as heating to heat and fire. In both these ways the active intellect can be compared to the intelligible object as perfection is to the perfectible, and as act is to potentiality. Now a subject is made perfect and receives its perfection at one and the same time, as the reception of what is actually visible synchronizes with the reception of light in the eye. Therefore the passive intellect receives the intelligible object and the active intellect together; and the more numerous the intelligible objects received, so much the nearer do we come