In De Anima II 12 and III 1, Aristotle argues that the common sensibles (movement, rest, figure, magnitude and number)are perceived by each special sense only per accidens and they are perceived per se by a common sense. To understand Aristotle's theory of the common sense consistently, however, we must answer the following three questions. The first is whether the common sense is an independent faculty of the special senses or not. This needs consideration , because at the beginning of De Anima III 1 Aristotle denies that there is any sence faculty or any sense organ other than those of five special senses. The common sense is a part of the perceptual faculties of the primary (central) sense organ. Likewise, in the case of the special senses, their perceptions are achieved ultimately in the primary sense organ. Then, the faculty of perception which belongs to the primary sense organ is also contained in the special senses. Accordingly, for the common sense, we don't need any sense organ other than those needed for the special senses. In a way, the special senses as a whole contain the common sense. The second question is as follows : Aristotle thinks all the senses are the faculties receiving sensible forms, but what are the forms of the common sensibles? In De Anima 424a17-b3 and 426a27-b7, Aristotle insists that the sense is some sort of ratio(logos), and the former passage suggests that sensible forms are some type of ratios as well, This suggestion is confirmed by the arguments in De Sensu. Then, it is possible to take the forms of the common sensibles as some type of ratios. The interpretation above enables us to construe the form of magnitude as the external ratio of an extension of some object to the extensions of other objects, and the form of figure as the internal ratio between some parts of the extension of an object. The third question is how we can defend the commonness of the common sensibles to the special senses against G. Berkeley's arguments which deny the commonness of magnitude and of figure to sight and touch. If we regard the forms of the common sensibles not as extentions as such but as some type of ratios, we can defend the commonness. Having identified magnitude with extention, Berkeley puts two points. (1) The visible objects(colour, light)and the tangible objects (solidity, resistance)are entirely different, therefore, there are fundamental differences between the visible magnitude and the tangible magnitude and between the visible figure and the tangible figure. (2) The tangible extension(i. e. magnitude) is invariably the same, but the visible extension(i. e. magnitude)varies as you approach or recede. If we deny the identity of magnitude with extension and consider(with Aristotle)that the magnitude is a sort of ratio, then these two points are clarified. For, first, although the visible extension and the tangible extension is radically different, the ratio of some extension to other extensions can be common to sight and touch. And, secondly, it is true that the visible extension of the same object changes according to its distance, but it is possible that in different perspectives the changing extension refers to the same ratio. Thus, we can defend the commonness of the common sensibles against Berkeley's arguments.
