This dogma of universal determinism was combined in the Stoical system with an equally outspoken materialism. The capacity for either acting or being acted on was, according to Plato, the one convincing evidence of real existence; and he had endeavoured to prove that there is such a thing as mind apart from matter by its possession of this characteristic mark.[28] The Stoics simply reversed his argument. Whatever acts or is acted on, they said, must be corporeal; therefore the soul is a kind of body.[29] Here they only followed the common opinion of all philosophers who believed in an external world, except Plato and Aristotle, while to a certain extent anticipating the scientific automatism first taught in modern times by Spinoza, and simultaneously revived by various thinkers in our own day. To a certain extent only; for they did not recognise the independent reality of a consciousness in which the mechanical processes are either reflected, or represented under a different aspect. And they further gave their theory a somewhat grotesque expression by interpreting those qualities and attributes of things, which other materialists have been content to consider as belonging to matter, as themselves actual bodies. For instance, the virtues and vices were, according to them, so many gaseous currents by which the soul is penetrated and shaped—a materialistic rendering of Plato’s theory that qualities are distinct and independent substances.[30] We must mention as an additional point of contrast between the Stoics and the subsequent schools which they most resembled, that while these look on the soul as inseparable from the body, and sharing its fortunes from first to last, although perfectly distinct from it in idea, they emphasised the antithesis between the two just as strongly as Plato, giving the soul an absolutely infinite power of self-assertion during our mortal life, and allowing it a continued, though not an immortal, existence after death.[31]