The Stoic theory of Ethics is entirely rooted in the proposition, that only virtue is a good and only vice an evil. This proposition, however, frequently brought the Stoics into collision with current views; nor was it without its difficulties for their own system. In the first place, virtue is made to depend for its existence upon certain conditions, and to lead to certain results, from which it is inseparable. These results, we have already seen, [699] were included by the Stoics in the list of goods. Moreover, virtue is said to be the only good, because only what is according to nature is a good, and rational conduct is for man the only thing according to nature. But can this be so absolutely and unconditionally stated? According to the Stoic teaching the instinct of self-preservation being the primary impulse, does not this instinct manifestly include the preservation and advancement of outward life? The Stoics, therefore, could not help including physical goods and activities among things according to nature—for instance, health, a right enjoyment of the senses, and such like. [700] Practically, too, the same admission was forced upon them by the consideration [701] that, if there is no difference in value between things in themselves, rational choice—and, indeed, all acting on motives—is impossible. At the same time, they reject the notion that what is first according to nature must therefore be perfect or good, just as in theory they allow that the source of knowledge, but not truth itself, is derived from the senses. When man has once recognised the universal law of action, he will, according to their view, think little of what is sensuous and individual, and only look upon it as an instrument in the service of virtue and reason. [702] [(1) Secondary goods.]