The essay consists of two parts. In the first he shows that miracles are incapable of proof by testimony. Belief is in proportion to evidence. Evidence rests on sensational experience. Accordingly the testimony to the uniformity of nature being universal, and that which exists in favour of the occurrence of a miracle, or violation of the laws of nature, being partial, the former must outweigh the latter. In the second he shows, that if this is true, provided the testimony be of the highest kind, much more will it be so in actual cases; inasmuch as no miracle is recorded, the evidence for which reaches to this high standard. He explains the elements of weakness in the evidence; such as the predisposition of mankind to believe prodigies, forged miracles, the decrease of miracles with the progress of civilization, the force of rival testimony in disproof of them, which he illustrates by historic examples, such as the alleged miracles of Vespasian, Apollonius, and the Jansenist Abbé Paris.(481) The conclusion is, that miracles cannot be so shown to occur as to be used as the basis of proof for a revelation; and that a revelation, if believed, must rest on other evidence. The argument accordingly is briefly, that testimony cannot establish a fact which contradicts a law of nature; the narrower induction cannot disprove the wider. The reasoning has been used in subsequent controversy(482) with only a slight increase of force, or alteration of statement. The great and undeniable discoveries of astronomy had convinced men in the age of Hume of the existence of an order of nature; and modern discovery has not increased the proof of this in kind, though it has heightened it in degree, by showing that as knowledge spreads the range of the operation of fixed law is seen to extend more widely; and apparent exceptions are found to be due to our ignorance of the presence of a law, not to its absence. The statement of the difficulty would accordingly now be altered by the introduction of a slight modification. Instead of urging that testimony cannot prove the historic reality of the fact which we call a miracle, the assertion would be made that it can only attest the existence of it as a wonder, and is unable to prove that it is anything but an accidental result of an unknown cause. A miracle differs from a wonder, in that it is an effect wrought by the direct interposition of the Creator and Governor of nature, for the purpose of revealing a message or attesting a revelation. That testimony can substantiate wonders, but not distinguish the miracle from the wonder, is the modern form of the difficulty.