We discover nothing in this refutation of skepticism calculated to destroy it. Even supposing all to admit the principle of causality, which all do not admit, what aid can he draw from this principle, when he makes the work of that very understanding, which must make use of it, the only criterion? If causality be the only criterion of truth, the understanding is isolated, and cannot, in the order of effects, take one step beyond what it has itself produced; and, in the order of causes, it cannot ascend higher than itself; for, were it so to ascend, it would know things not made by itself, would know its own cause. With this supposition the skeptics must triumph; cognition is confined to the internal world, to simple appearances; and when one would go out of these, he stumbles against the only criterion which opposes the cognition of all _not made_ by the understanding itself. We do then see reality, but are separated from it by an impassable abyss. The world in itself may be any thing we choose to suppose it; but with respect to us, it will be nothing. This law applies to every intelligence, so that vitality can only be known by the first cause. These consequences, inadmissible as they are, if we would not throw ourselves unreservedly into the tide of skepticism, are nevertheless inevitable in Vico's system. An original way truly of combatting skepticism, thus to throw open its widest gates! CHAPTER XXXI. CONTINUATION OF THE SAME SUBJECT. 304. If the Neapolitan philosopher's criterion be anywhere admissible, it can only be in ideal truths; for as these are absolutely cut off from existence, we may well suppose them to be known even by an understanding which has not in reality produced them. So far as known by the understanding they involve no reality, and consequently no condition that exacts any productive force not referable to a purely ideal order. In this order the human reason seems really to produce. If we, for example, take geometry, we shall readily perceive that, even in its profoundest parts and in its greatest complications, it is only a kind of intellectual construction, wherein that only is to be found which reason has placed there. Reason it is which by force of perseverance has succeeded in uniting elements and so disposing them as to attain that wonderful result, of which it may say with truth: _this is my work_. If we carefully observe the development of the science of geometry, we shall perceive that the extended series of axioms, theorems, problems, demonstrations and solutions, begins with a few postulates, and that it goes on with the aid of the same, or others discovered by reason according to the demands of necessity or utility.