Algazel says: "You should know that the internal senses are five also, i.e., the common sense, the imaginative faculty, the thinking faculty, the estimative faculty and the memorizing faculty. The common sense is the sense from which all those five derive, to which each of their impressions is announced, and in which they all are joined, and thus they are all, as it were, filling it up. For if there was nothing in us in which white and sound were joined, we would not know that this white object is that whose sound we have heard. For neither the eye nor the ear grasps the conjunction of whiteness and sound."

Original Latin

Five External Senses, and Five Internal Senses

The germ idea of 'internal senses' by which information is processed is already in Galen. Galen, a doctor, attributed different powers to different parts of the brain. The symmetry between five internal senses and five external senses appears to have been invented by Avicenna. He states this most clearly in the philosophical summa that he wrote in Persian, the Danesh Nameh (pp. 62-4). It is on this source that Algazel, who read Persian as fluently as he read Arabic, clearly relied.

The combination of a physiological and a psychological account of the faculties is noticeable here. Understanding and judgment are reached through a sequence of events: sense perceptions are gathered together in one place (the common sense), and stored, as it were, in the treasury of the imagination. At the same time, impressions that cannot be perceived directly (such as the danger of the wolf) are received and stored in the treasury of the memory; these are called 'intentions' in Latin, and they form the basis for judgment in men and beasts (the estimative power). To bring out the importance of the 'thinking' (cogitatio) that is unique to man, Algazel placed it at the end of his account (in Avicenna's Danesh Nameh it precedes the account of memory). Algazel does not call this power 'intellect' or 'reason'. It draws from the treasuries of both the imagination and the memory and forms judgments.

Inside the Brain

Physiologically, the imagination is described as being in the front part of the brain and the memory in the back, so that the thinking power, placed in the middle, can draw from both. This account conforms to what we find in Qusta ibn Luqa's De differentia spiritus et animae (the Latin translation of which usually accompanied Aristotle's De anima in the collections of the Libri naturales of the medieval universities). But Qusta adds that there is a little worm-like organ (he is probably thinking of the pineal gland) which allows the memories to pass from the back part to the rational area of the brain so that one can think with them, and form judgments.

The note-like quality of Algazel's account entails some jumps in the argument. For example 'intentions' are mentioned under the memorizing power, without having been previously defined. A fuller account is found In Avicenna's De anima as part of his Arabic philosophical encyclopedia. For example, the coming together of whiteness and sound in the common sense is described more picturesquely as the perception of a white man singing and dancing (De anima, part 4, ed. S. Van Riet, p. 2, lines 21-22).

So, what are these faculties again?

Algazel's vacillation between describing the internal senses as 'senses' and as 'powers' suggests an uncertainty as to whether they are substantial entities or attributes of something else. Later on he concludes that all these internal senses (including the thinking power) are instruments of the soul. The Latin translator sometimes amplifies the Arabic text, adding 'five' (i.e. the five previously described external senses) as the number of senses deriving from and returning to the common sense. He also makes the Arabic more vivid, imagining the senses as messengers bringing news back to the common sense, by using the verb renuntiare where the Arabic uses the common word for 'return' (raja'a).