Next comes the turn of scientific research or reasoning. This, he maintains, leads to doubt of everything, for the highest of all principles do not admit of proof; we are not certain, moreover, of the reliability of memory. It is impossible to parry this attack upon the scientific method in so far that it is of course impossible to prove the reliability of memory without pre-supposing the reliability of the memory which is to be proved. But of the indirect proofs of the reliability of memory provided by human experience Lamennais does not say a single word.
He touches provisionally upon the subject of complete doubt. Complete doubt would lead to complete insanity. The spirit of self-preservation compels us to believe and to act according to our belief. It is, in the Abbé's opinion, this want of ability to doubt, or the knowledge that one will, if one doubts, be regarded by other men as ignorant or mad, which forms the foundation of all human certainty. Common consent (_sensus communis_) thus becomes for us the seal of truth, and there is no other. Difference of opinion at once begets uncertainty. A principle or a fact is more or less certain according as it is more or less universally accepted and borne witness to. Hence Lamennais' definition of a science is: A science is a collection of thoughts and facts on which all men are agreed. Though his standpoint is a different one, he resembles the English empirical philosophers of a later day in refusing even to such a science as geometry any foundation but that of common consent. The fact that many a mistaken scientific conclusion has been taken for truth is due, he believes, to the circumstance that science has reached only a small number of human beings. What, he exclaims, are a few hundred savants compared with the whole human race! He strangely enough forgets that the human race has never unanimously accepted a single scientific truth previous to its discovery by men of science, in fact has never shown original unanimity in any belief.
Lamennais asks: When two persons disagree, what do they do after they have in vain attempted to over-persuade one another? and he answers: They appeal to arbitration. But what is arbitration? Arbitration is _authority_, and this authority declares with which of the differing opinions certainty, or if not certainty, at least probability rests. The fact that the arguments of reason, as such, only create doubt, and the fact that the strongest proof of the mistakenness of an assertion always is: "You are the only one who thinks thus," direct us to the _principle of authority_ as the only true and final principle.