Selection from – Ethics – Part II. On the Nature and Origin of the Mind (Page 2)
Spinoza's Words: (on adequate ideas)
DEFINITION IV. By an adequate idea, I mean an idea which, in so far as it is considered in itself, without relation to the object, has all the properties or intrinsic marks of a true idea.
Explanation.—I say intrinsic, in order to exclude that mark which is extrinsic, namely, the agreement between the idea and its object .
Comment:
Adequate ideas have for Spinoza great significance. He makes much use of the concept further on in this second book of the Ethics. Here he drops it in as a definition emphasizing that adequate ideas are true in themselves and not simply a mental image (an inadequate idea) of an object.
Elsewhere (in Part III) Spinoza writes "By an adequate cause, I mean a cause through which its effect can be clearly and distinctly perceived. By an inadequate or partial cause, I mean a cause through which, by itself, its effect cannot be understood." For Spinoza the word "adequate" is to be taken as something clearly understood.