The origin of all perceptions (φαντασίαι) may be referred to the action of some object (φανταστὸν) on the soul, [128] the soul at birth resembling a blank page, and only receiving definite features by experience from without. [129] By the elder Stoics, this action of objects on the soul was regarded as grossly material, Zeno defining a perception to be an impression (τύπωσις) made on the soul, [130] and Cleanthes took this definition so literally as to compare the impression on the soul to the impression made by a seal on wax. [131] Being himself a very exact pupil of Zeno, Cleanthes probably rendered the views of Zeno correctly in this comparison. The difficulties of this view were recognised by Chrysippus, who accordingly defined a perception to be the change (ἑτεροίωσις) produced in the soul by an object, or, more accurately, the change produced thereby in the ruling part of the soul; [132] and whereas his predecessors had only considered sensible things to be objects, he included among objects conditions and activities of the mind. [133] The mode, however, in which the change was produced in the soul did not further engage his attention. [(b) Conceptions formed from perceptions.] It follows, as a necessary corollary from this view, that the Stoics regarded sensation as the only source of all perceptions: the soul is a blank leaf, sensation is the hand which fills it with writing. But this is not all. Perceptions give rise to memory, repeated acts of memory to experience, [134] and conclusions based on experience suggest conceptions which go beyond the sphere of direct sensation. These conclusions rest either upon the comparison, or upon the combination of perceptions, or else upon analogy; [135] some add, upon transposition and contrast. [136] The formation of conceptions by means of these agencies sometimes takes place methodically and artificially, at other times naturally and spontaneously. [137] [(α) Κοιναὶ ἔννοιαι formed naturally.] In the latter way are formed the primary conceptions, προλήψεις or κοιναὶ ἔννοιαι, which were regarded by the Stoics as the natural types of truth and virtue, and as the distinctive possession of rational beings. [138] To judge by many expressions, it might seem that by primary conceptions, or κοιναὶ ἔννοιαι, [139] innate ideas were meant; but this view would be opposed to the whole character and connection of the system. In reality, these primary conceptions, or κοιναὶ ἔννοιαι, are only those conceptions which, by reason of the nature of thought, can be equally deduced by all men from experience; even the highest ideas, those of good and evil, having no other origin. [140] The artificial formation of conceptions gives rise to knowledge, which is defined by the Stoics to be a fixed and immovable conception, or system of such conceptions. [141] Persistently maintaining, [(β) Knowledge formed artificially.] on the one hand, that knowledge is a