"A definition, if it is to be called perfect, must explain the inmost essence of a thing, and must take care not to substitute for this any of its properties. In order to illustrate my meaning, without taking an example which would seem to show a desire to expose other people's errors, I will choose the case of something abstract, the definition of which is of little moment. Such is a circle. If a circle be defined as a figure, such that all straight lines drawn from the center to the circumference are equal, every one can see that such a definition does not in the least explain the essence of a circle, but solely one of its properties. Though, as I have said, this is of no importance in the case of figures and other abstractions, it is of great importance in the case of physical beings and realities, for the properties of things are not understood so long as their essences are unknown. If the latter be passed over, there is necessarily a perversion of the succession of ideas which should reflect the succession of nature, and we go far astray from our object."

“Humanam impotentiam in moderandis et coercendis affectibus servitutem voco; homo enim affectibus obnoxius sui juris non est sed fortunæ in cujus potestate ita est ut sæpe coactus sit quanquam meliora sibi videat, deteriora tamen sequi.”

“Meum institutum non est verborum significationem sed rerum naturam explicare”

“Hic conatus aliquid agendi et etiam omittendi ea sola de causa ut hominibus placeamus, vocatur ambitio præsertim quando adeo impense vulgo placere conamur ut cum nostro aut alterius damno quædam agamus vel omittamus; alias humanitas appellari solet.”

“veritas norma sui et falsi est”

“Ordo et connexio idearum idem est ac ordo et connexio rerum”

“[B]y Natura naturans we must understand what is in itself and is conceived through itself, or such attributes of substance as express an eternal and infinite essence, that is … God, insofar as he is considered as a free cause. But by Natura naturata I understand whatever follows from the necessity of God's nature, or from God's attributes, that is, all the modes of God's attributes insofar as they are considered as things which are in God, and can neither be nor be conceived without God.”