2. The other is the tendency to under-estimate the importance of the opinions that have disappeared from sight, or which we know only in the form and to the extent of their quotation by their opponents. If we were to trust the histories that are commonly current, we should believe that there was from the first a body of doctrine of which certain writers were the recognized exponents; and that outside this body of doctrine there was only the play of more or less insignificant opinions, like a fitful guerilla warfare on the flanks of a great army. Whereas what we really find on examining the evidence is, that out of a mass of opinions which for a long time fought as equals upon equal ground, there was formed a vast alliance which was strong enough to shake off the extremes at once of conservatism and of speculation, but in which the speculation whose monuments have perished had no less a share than the conservatism of which some monuments have survived. * * * * * This survey of the nature of the evidence enables us to determine the method which we should follow. We can trace the causes and we can see the effects; but we have only scanty information as to the intermediate processes. If the evidence as to those processes existed in greater mass, if the writings of those who made the first tentative efforts to give to Christianity a Greek form had been preserved to us, it might have been possible to follow in order of time and country the influence of the several groups of ideas upon the several groups of Christians. This method has been attempted, with questionable success, by some of those who have investigated the history of particular doctrines. But it is impossible to deprecate too strongly the habit of erecting theories upon historical quicksands; and I propose to pursue the surer path to which the nature of the evidence points, by stating the causes, by viewing them in relation to the effects, and by considering how far they were adequate in respect of both mass and complexity to produce those effects.