system of artificial conceptions, impossible without a logical process, they must, on the other hand, have felt it imperative from this platform that knowledge should agree in its results with primary conceptions, [142] agreement with nature being in every department their watchword. For them it was as natural to derive support for their system from a supposed agreement with nature, as it was easy for their opponents to show that this agreement with nature was imaginary, and that many of their assertions were wholly opposed to general opinions. [143] [(c) Relation of perceptions and conceptions.] Perceptions, and the conclusions based upon them, [144] being thus, according to the Stoics, the two sources of all notions, the further question arises, How are these two sources related to each other? It might have been expected that only perceptions would be stated to be originally and absolutely true, since all general conceptions are based on them. Nevertheless, the Stoics are far from saying so. Absolute certainty of conviction they allow only to knowledge, and therefore declare that the truth of the perceptions of the senses depends on their relation to thought. [145] Truth and error do not belong to disconnected notions, but to notions combined in the form of a judgment, and a judgment is produced by an effort of thought. Hence sensations, taken alone, are the source of no knowledge, knowledge first arising when the activity of the understanding is allied to sensation. [146] Or, starting from the relation of thought to its object, since like can only be known by like according to the well-known adage, the rational element in the universe can only be known by the rational element in man. [147] But again, the understanding has no other material to work upon but that supplied by sensation, and general conceptions are only obtained from sensation by conclusions. The mind, therefore, has the capacity of formally working up the material supplied by the senses, but to this material it is limited. Still, it can progress from perceptions to notions not immediately given in sensation, such as the conceptions of what is good and of God. And since, according to the Stoic teaching, material objects only possess reality, the same vague inconsistency may be observed in their teaching as has been noticed in Aristotle [148]—reality attaching to individuals, truth to general notions. This inconsistency, however, is more marked in their case than in that of Aristotle, because the Stoics so far adhere to the Cynic nominalism [149] as to assert that no reality attaches to thought. [150] Such an assertion makes it all the more difficult to understand how greater truth can be attributed to thought, unreal as it is said to be, than to sensations of real and material objects. Do we then ask in what the peculiar character of thought consists, the Stoics, following Aristotle, reply that in thought the idea of universality is added to