down the opposite principles.
The fifth mode, that one namely which arises from the reciprocal nature of proofs, is capable of application whenever the proof of the truth which we are looking for supposes, as a necessary preliminary, our belief in that truth; for instance, if, after we have proved the porosity of bodies by their evaporations, we return and prove the evaporations by the porosity.
XI. These Sceptics then deny the existence of any demonstration, of any test of truth, of any signs, or causes, or motion, or learning, and of anything as intrinsically or naturally good or bad. For every demonstration, say they, depends either on things which demonstrate themselves, or on principles which are indemonstrable. If on things which demonstrate themselves, then these things themselves require demonstration; and so on ad infinitum. If on principles which are indemonstrable, then, the very moment that either the sum total of these principles, or even one single one of them, is incorrectly urged, the whole demonstration falls instantly to pieces. But if any one supposes, they add, that there are principles which require no demonstration, that man deceives himself strangely, not seeing that it is necessary for him in the first place to establish this point, that they contain their proof in themselves. For a man cannot prove that there are four elements, because there are four elements.
Besides, if particular proofs are denied in a complex demonstration, it must follow that the whole demonstration is also incorrect. Again, if we are to know that an argument is really a demonstrative proof, we must have a test of truth; and in order to establish a test, we require a demonstrative proof; and these two things must be devoid of every kind of certainty, since they bear reciprocally the one on the other.
How then is any one to arrive at certainty about obscure matters, if one is ignorant even how one ought to attempt to prove them?