The principle of sufficient reason is simple as this: For any given fact or true proposition, there exists a reason behind, although some of those reasons we have not yet come to know, or may never know; or in other words: “Things exist for a reason”.
Once this principle is applied to a field of inquiry, it follows that that field could be understandable. If we apply it to the field of possible human experience, we assume that human life is not absurd, but rational. And if some proposition do not have any possible reasons (i.e., “God exists”) in its supports, then the proposition cannot be true.
On the constructive side, when in order to explain something, several conditions must obtain: (1) There must be a subject, or someone who is seeking an explanation; (2) There must be something that that person wants to explain; (3) In order for the explanation to be legitimate, it must necessary, not arbitrary or merely tentative.
And Schopenhauer argues there must be some presuppositions for this principle to be applicable: (1) the subject-object distinction, which when present, entails the appearance always of both a subject and an object in relation to each other, and (2) links between the components of the explanation that are necessary, for if the linkages are not necessary, then there is no true explanation. If there is no subject who seeks an explanation, then there can be no object of awareness to be explained. When there are subjects who are aware of objects, then these objects of awareness are “representations” (vorstellung), or mental images. Conversely, if there is an object, or representation, then there must be a subject in which that representation appears.
This is the “root” of the principle of sufficient reason which Schopenhauer wants to exhaust all kinds of necessary connection between subject and object backing the principle. However, he came to believe that the foundation had not yet been well defined, for the preceding history of philosophy always contained the uncritical assumption that “reason” is a single concept, equally applicable in all kinds of circumstances. Schopenhauer maintains, though, that the concept of “reason” is like the concept of “triangle”: if there is a triangle, it exclusively must be equilateral, isosceles, or scalene. Thus, there must be different “kinds” of reason.
Schopenhauer accordingly identifies four kinds of reason and four kinds of corresponding objects that can operate within explanation exclusively. We can be interested in:
(1) explaining the conditions of physical things, for which causal explanation is appropriate
(2) explaining relationships between concepts, for which logical explanation is appropriate
(3) explaining relationships between numbers, formulae, or geometrical figures, for which mathematical/geometrical explanation is appropriate
(4) explaining motivations, inner drives, or the presence of subjective states of mind, for which motive-referring, psychological explanation is appropriate.
Once we choose the kind of object to be explained, we must hold fast to the kind of explanation that is appropriate to that kind of object and resist introducing styles of reasoning that are appropriate to other kind of object. I.e., one would not genuinely explain why a person felt that he or she should pay a debt and acted accordingly to pay the debt, by referring to neurological firings in the person’s brain that correlated with the action. This example involves confusing causal explanation with psychological explanation.
With this foundation, Schopenhauer criticizes the illegitimacy in the works of many philosophers, especially in the acceptance of the ontological argument for God’s existence, which he regards as based on exactly the kind of confusion he is urging his readers to guard against. The ontological argument starts with mere conceptual definitions (viz., “God is a being than which no greater can be conceived”) and concludes dramatically by positing an existent being (viz., “God exists”). According to Schopenhauer, this confuses logical explanation with causal explanation.
Schopenhauer is emphatic about restricting the scope of the principle, and argues that it can be used to refer to the entire set of necessary connections for human experience that Kant describes. For Kant, these are space, time, and twelve pure concepts of the understanding. For Schopenhauer, there are space, time, and the single concept of understanding that he continues to recognize, namely, the concept of causality. In either inventory, these forms are ascribed to the human mind exclusively, and they define our capacity to know things scientifically. They are not features of the world as it is in itself, but are only features of our human way of knowing. Schopenhauer consequently refers to the principle of sufficient reason as one that generates for us the illusory world of space, time, and objects that interact with each other in a thorough going causal mechanism.