Let us set ourselves before this reality. Leaving aside all conceptions of religion in general, let us consider the various religions in their concrete reality, and attempt to disengage that which they have in common; for religion cannot be defined except by the characteristics which are found wherever religion itself is found. In this comparison, then, we shall make use of all the religious systems which we can know, those of the present and those of the past, the most primitive and simple as well as the most recent and refined; for we have neither the right nor the logical means of excluding some and retaining others. For those who regard religion as only a natural manifestation of human activity, all religions, without any exception whatsoever, are instructive; for all, after their manner, express man, and thus can aid us in better understanding this aspect of our nature. Also, we have seen how far it is from being the best way of studying religion to consider by preference the forms which it presents among the most civilized peoples.[26] But to aid the mind in freeing itself from these usual conceptions which, owing to their prestige, might prevent it from seeing things as they really are, it is fitting to examine some of the most current of the definitions in which these prejudices are commonly expressed, before taking up the question on our own account. I One idea which generally passes as characteristic of all that is religious, is that of the supernatural. By this is understood all sorts of things which surpass the limits of our knowledge; the supernatural is the world of the mysterious, of the unknowable, of the un-understandable. Thus religion would be a sort of speculation upon all that which evades science or distinct thought in general. "Religions diametrically opposed in their overt dogmas," said Spencer, "are perfectly at one in the tacit conviction that the existence of the world, with all it contains and all which surrounds it, is a mystery calling for an explanation"; he thus makes them consist essentially in "the belief in the omnipresence of something which is inscrutable."[27] In the same manner, Max Müller sees in religion "a struggle to conceive the inconceivable, to utter the unutterable, a longing after the Infinite."[28]