In all these researches the Stoics were striving to find firm ground for a scientific process of proof. Great as was the value which they attached to such a process, they nevertheless admitted, as Aristotle had done before, that everything could not be proved. Here was their weak point. Instead, however, of strengthening this weak point by means of induction, and endeavouring to obtain a more complete theory of induction, they were content with conjectural data, sometimes self-evident, at other times depending for their truth on the truth of their inferences. [252] Thus, their theory of method, like their theory of knowledge, ended by an ultimate appeal to what is directly certain. [(2) Its value.] No very high estimate can therefore be formed of the formal logic of the Stoics. Incomplete as our knowledge of that logic may be, still what is known is enough to determine the judgment absolutely. We see indeed that the greatest care was expended by the Stoics since the time of Chrysippus in tracing the forms of intellectual procedure into their minutest ramifications, and referring them to fixed types. At the same time, we see that the real business of logic was lost sight of in the process, the business of portraying the operations of thought, and giving its laws, whilst the most useless trifling with forms was recklessly indulged in. The Stoics can have made no discoveries of importance even as to logical forms, or they would not have been passed over by writers ever on the alert to note the slightest deviation from the Aristotelian logic. Hence the whole contribution of the Stoics to the field of logic consists in their having clothed the logic of the Peripatetics with a new terminology, and having developed certain parts of it with painful minuteness, whilst they wholly neglected other parts, as was the fate of the part treating of inference. Assuredly it was no improvement for Chrysippus to regard the hypothetical rather than the categorical as the original form of inference. Making every allowance for the extension of the field of logic, in scientific precision it lost more than it gained by the labours of Chrysippus. The history of philosophy cannot pass over in silence this branch of the Stoic system, so carefully cultivated by the Stoics themselves, and so characteristic of their intellectual attitude. Yet, when all has been said, the Stoic logic is only an outpost of their system, and the care which was lavished on it since the time of Chrysippus indicates the decline of intellectual originality. CHAPTER VI. THE STUDY OF NATURE. FUNDAMENTAL POSITIONS.