The problem, in a sense, resolves around the notion of credulity. What shall we believe? Boyle makes some distinctions between what he has seen with his own eyes and what other people report to have seen. Thus, he mentions "a very experienced and sober gentleman, who is much talked of" who cured cancer of the female breast "by the outward application of an indolent powder, some of which he also gave me." But, he adds cautiously, he has not yet "had the opportunity to make trial of it."[52] Clearly, since he cannot make the trial himself, Boyle withholds judgment, even though the material came from a "very experienced" gentleman. Or again, he talks about "sober travelers" who made certain claims regarding the treatment of poisons. But, he says, "having not yet made any trial of this my self, I dare not build upon it."[53] There are numerous such instances, scattered throughout his works, where he reports an alleged cure but specifically indicates his own mental reservations. Clearly, he is quite cautious in accepting the statements of others, even though they were "sober" or "experienced" or even "judicious." On the other hand, he is extremely uncritical when he himself uses the term "cure" and when he attributes cures to particular medicines. His skepticism he indicates in references, for example, to Paracelsus and van Helmont. Their specific remedy against "the stone," he says, and their claims that they can reduce stones to "insipid water, is so strange (not to say incredible) that their followers must pardon me, if I be not forward to believe such unlikely things, til sufficient experience hath convinced me of their truth."[54] Here, of course, we see further a feature of critical acumen. A claim is made, but if this claim runs counter to Boyle's own accepted body of knowledge, or to logical doctrines derived from other directions, mere assertion cannot carry conviction. "Sufficient experience" must play its part, and just what constitutes "sufficient" we are not quite sure. In judging the effectiveness of a remedy or the credibility of a statement, one of the most important weapons was _analogy_. Direct observation of a phenomenon was good. Next best was direct observation of some _analogous_ phenomenon whereby one body acted upon another to alter its properties or induce significant changes. Boyle drew his analogies largely from chemistry, but he had no hesitation in applying them to medicine.