Two degrees are to be distinguished in the true or adequate knowledge of the intellect:  rational knowledge attained through inference, and intuitive, self-evident knowledge; the latter has principles for its object, the former that which follows from them.  Instead of operating with abstract concepts the reason uses common notions, notiones communes.  Genera do not exist, but, no doubt, something common to all things.  All bodies agree in being extended; all minds and ideas in being modes of thought; all beings whatever in the fact that they are modes of the divine substance and its attributes; “that which is common to all things, and which is equally in the part and in the whole, cannot but be adequately conceived.”  The ideas of extension, of thought, and of the eternal and infinite essence of God are adequate ideas.  The adequate idea of each individual actual object involves the idea of God, since it can neither exist nor be conceived apart from God, and “all ideas, in so far as they are referred to God, are true.”  The ideas of substance and of the attributes are conceived through themselves, or immediately (intuitively) cognized; they are underivative, original, self-evident ideas.

There are thus three kinds, degrees, or faculties of cognition—­sensuous or imaginative representation, reason, and immediate intuition.  Knowledge of the second and third degrees is necessarily true, and our only means of distinguishing the true from the false.  As light reveals itself and darkness, so the truth is the criterion of itself and of error.  Every truth is accompanied by certainty, and is its own witness (II. prop. 43, schol.).—­Adequate knowledge does not consider things as individuals, but in their necessary connection and as eternal sequences from the world-ground.  The reason perceives things under the form of eternity:  sub specie aeternitatis (II. prop. 44, cor. 2).

In his theory of the emotions, Spinoza is more dependent on Descartes than anywhere else; but even here he is guided by a successful endeavor after greater rigor and simplicity.  He holds his predecessor’s false concept of freedom responsible for the failure of his very acute inquiry.  All previous writers on the passions have either derided, or bewailed, or condemned them, instead of investigating their nature.  Spinoza will neither denounce nor ridicule human actions and appetites, but endeavor to comprehend them on the basis of natural laws, and to consider them as though the question concerned lines, surfaces, and bodies.  He aims not to look on hate, anger, and the rest as flaws, but as necessary, though troublesome, properties of human nature, for which, as really as for heat and cold, thunder and lightning, a causal explanation is requisite.—­As a determinate, finite being the mind is dependent in its existence and its activity on other finite things, and is incomprehensible without them; from its involution in the general course