The term reason – from latin, ratio – derives from the expression used by accountants as referring to the ledger. Since in such account book the accounting effects were recorded according to each specificity, the philosophers began to used it to indicate the discursive intellective faculty of man. Such appropriation – it is worth mentioning – should not surprise since Philosophy in its beginnings resorted from popular vernacular most of the vocabulary with which its discursive universe was built. Hodiernal philosophers turn to greek and latin languages instead of the popular vocabulary as a resource and we therefore have lost the notion of the origin of such terms, which seem specially construct to point out intellectual intentionalities.
In addition, this term has yet other meanings such as the cause that motivates an act. Thus, one can say that the reason of being of something is the cause of its existence. It is also used to signify quiddity, or nature, or specie, or even the form of things. Actually, considered in latu sensu, the term denotes that whereby the entity is what it is. And also indicates the order of the essence of something (a broader meaning), the order of existence, the order of its intelligibility or of its truth. The term ratio corresponds to the greek word logos.
The expression sufficient reason is widely used in Philosophy as referring to that of what is required from something to be what it is in the order in which it is, whilst it is called insufficient reason when such requirement is not accomplished. Both terms are used in absolute and relative senses. In the former case, when it is fully complied with the being; in the latter, when such compliance is only partial.
Since the causes of a thing are intrinsic or extrinsic, sufficient reason was divided similarly. Thus the formal and the material causes offered an intrinsic reason to the being, and the efficient, final and exemplar, an extrinsic reason.
Since nothingness nothing can and consequently achieves nothing (since to do implies potentiality) nor transmute in nothing since it is nothing, that that it is must have a sufficient cause to be what it is and not be what it is not. Therefore the classical enunciate of the principle of sufficient reason: No thing can be without its sufficient reason. It is called a principle due to its necessity and absolutism, for how can something be if there was no reason for it to be?
In the essence is included not only the essential notes, but also the properties and even some accidents. Therefore the classical enunciate: Anything that has a certain essence must have a sufficient reason for such essence. Here is an example of the use of the principle of sufficient reason regarding the order of essence.
Also that that exists has a sufficient reason of its existence. It is the application to the order of existence.
What is known has a sufficient reason for which can be known. It is the application to the order of intelligibility, which can also be expressed as: every true judgement has a sufficient reason of its truth.
To sum up we can say:
Whatever is, exists or can be understood has to have, intrinsical or extrinsically (in its emergence or its predisponence), partial or totally, a sufficient reason of its essence, of its existence or of its intelligibility.
That is the principle of sufficient reason.
However many philosophers, not knowing what it is and creating of it a caricature, present inappropriate arguments to oppose it. To deny it wholly or partially or to affirm it only partially has been the attitude taken by philosophers throughout the time.
It is important to clarify that the Greeks did not properly, but it was implicitly contained within the positive thought coming from Pythagoras, Socrates, Plato and Aristotle. The scholastics also did not formulate it, but it was precisely Leibnitz who formulated it as a foundation of Metaphysics, along with the principle of noncontradiction. Kant and the idealists considered it initially as a merely subjective principle by denying it an objective necessity.
Positivists and empiricists did not totally denied the principle of sufficient reason although considering it valid only in the field of phenomenons, for in the metaphysical field they affirm it impossible to establish its validity.
The positive and concrete thesis however is the affirmation of the validity both in regard to necessary entities as contingent ones, and so much as in the order of essence as in the existence and intelligibility.
The explanation already offered shows that this principle enunciates universal and necessarily, since the being is impossible without a reason of being; it is universal, since it refers to all species of being and it is necessarily convenient to all species of being, since without it no being would have a reason to be; it is evident in the order of essence (since all that is has an essence), in the order of existence (since it is impossible to exist that that has no reason to do so, and that that exists do so for it has a reason to do so). A thing is only intelligible whilst that is a reason of its intelligibility, for nothingness is unintelligible: if the being lacks cognoscible notes, how could one know it? On the other hand, a judgement is true in the proportion of its adequation; therefore for a judgment to be true is imperative to have a reason to be so. Without a reason of cognoscibility, we cannot know anything.
Therefore, it is an error to consider such principle merely regional and partially. And such error was committed by many philosophers – markedly modern ones who reduced it to a mere logical principle. They are precisely the same authors that claim that Logic is merely an expression of the way our mind works, without a possibility of a concrete and positive foundation in reality – to which man also belongs. They cannot distinguish that ideality is the nexus of ideal things and reality is the nexus of real things, and that that is also a ideality of reality and a reality of ideality.