Home Economics Aquinas’s theory of perception: an analytic reconstruction
The Three Ventricles
The sensus communis was found in the front part of the forward ventricle. In an Aristotelian fashion, the sensus communis is where the discrete impressions from the external sense faculties are combined. Like Aristotle before him and Aquinas afterwards, Avicenna adopts a ‘bundle view of sensations’, with the physical object being reducible to a combination of discrete sensations—Aristotle’s proper and common sensibles—coming from the different external sense faculties. This is, of course, the classic ‘heap theory’ of a physical object.
Avicenna placed both the sensus communis and the imaginatio in the foremost ventricle in the human head. Kemp writes: ‘all the sensory nerves were believed to be connected to the front ventricle, in the front of which the common sense was located.’43 Kemp also notes that, according to Avicenna, ‘the consistency of the front ventricle was more liquid and slippery than the back, so that sensation was rapidly received by the common sense but also quickly lost if the stimulus were removed’. The state of the liquid character of the frontmost part of the ventricle is contrasted with the state of the rear part of this ventricle: ‘the imagination at the back could retain images because it was drier.’ Kemp’s analysis is consistent with several puzzles found in reading Aquinas’s texts on the internal sense organs where Thomas uses the ‘moist’ description.
According to Avicenna, the sensory information from the front ventricle is passed through to the middle ventricle by means of a narrow passage, which had a gate-like mechanism: a small ‘worm-like organ known as the vermis’ In the middle ventricle there were, according to Kemp’s reading of Avicenna, two cognitive processes, what Aquinas later would call the vis aestimativa and the vis cogitativa. According to Kemp, the function of the vis cogitativa was to form images like ‘the golden mountain’, composed from discrete images or phantasms of a mountain and of gold. The workings of the vis aestimativa, according to Kemp’s reading of Avicenna, are like ‘implications’, which are either instinctively or are the results of what Kemp calls ‘associative learning’. Kemp illustrates this by using the familiar examples from discussions on the vis aestimativa: ‘a sheep will instinctively fear a wolf even if it has never encountered one before, since it can recognize the threat to it that is one of the implications of the appearance of the wolf. Also a dog will cringe in terror from a stick with which it has been previously beaten.’ 
Finally, the inner sense faculty of memory—memoria or vis memorativa—is found in the rear ventricle of the brain. Kemp’s reading of Avicenna suggests that the memory—what some translators of Aquinas call the ‘sense memory’ in order to distinguish it from the vis imaginativa—stores the phantasms from the vis aestimativa. In the case of human knowers, Aquinas would have the phantasms from the vis cogitativa stored in the vis memorativa. Humans do not have a vis aestimativa; on this point, Aquinas writes: ‘Therefore, the power which in other animals is called the natural aestimativa in humans is called the cogitativa’ (Summa Theologiae, I q. 78 a. 4).
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