=151.= With the mind-pictures (φαντασίαι, _visa_) which are derived from sensation we may now contrast the ‘notions’ (ἔννοιαι, _notiones_ or _intellegentiae_) which are derived from the combination of sensation and reasoning; the former correspond generally to ‘perceptions,’ the latter to ‘conceptions’ in the language of modern philosophy[51]. But each of the Stoic terms is also used in a wider sense which includes the other. The sensory pictures are inscribed upon the mind as upon a blank sheet from birth upwards; in this sense they may well be called ‘entries on the mind’ (ἔννοια from ἐν νῷ)[52]. On the other hand the conceptions may be called ‘rational mind-pictures’[53]; quite as much as the sensory mind-pictures they need the prudent assent of the will before they become ‘comprehensions,’ when they are once more units capable of entering into further combinations and becoming part of scientific knowledge. If then for the sake of clearness we use the modern terms, we may say that perceptions correspond generally to individual objects which have a real existence, whilst conceptions correspond to classes of things, which (according to the Stoics) have no real existence in themselves, but only a sort of existence in our minds. Thus the ‘ideas’ of Plato are all conceptions, subjectively but not objectively existent[54]. So far as our study has gone, all conceptions are based on perceptions: therefore all the elements of knowledge either come from sense and experience solely, or from sense and experience combined with reasoning[55]; and the most important reasoning process is that comparison of like perceptions which in this philosophy takes the place of induction[56]. [Sidenote: Preconceptions.]