This distinction is closely linked in Ibn Sina's philosophy...

This distinction is closely linked in Ibn Sina's philosophy to his important metaphysical claim that essence or quiddity can be distinguished from existence, and that existence in turn can be considered in either of its two modes: existence in concrete, singular things in the external world; or conceptual existence in one of the soul's sensible or intellectual faculties.

In al-Madkhal , Ibn Sina argues that logic differs from the other sciences because it considers not conceptual existence as such (this would be psychology), but rather the accidents or properties that belong to any quiddity by virtue of its being conceptualized by the mind. These properties, according to Ibn Sina, include such things as essential and accidental predication, being a subject or being a predicate, and being a premise or a syllogism.

It is these properties that allow the mind to connect concepts together in order to acquire knowledge of the unknown; they provide the foundation for the rules of reasoning and inference that logic studies.

They are moreover formal properties in the sense that, as properties belonging to all concepts in virtue of their mental mode of existence, they are entirely independent of the content of the thought itself; they are indifferent to the intrinsic natures of the quiddities which they serve to link together.

In the Ilahiyyat (Metaphysics) of al-Shifa', Ibn Sina introduces the terminology of first and second 'intentions' or concepts in order to express the relation between the concepts of these quiddities themselves - which are studied in the theoretical sciences - and the concepts of the states and accidents of their mental existence which logic studies: 'As you know, the subject matter of logical science is second intelligible intentions (al-ma'ani al-ma'qula al-thaniyya) which are dependent upon the primary intelligible intentions with respect to some property by which they lead from the known to the unknown' (Ilahiyyat Book 1, ch.

2,). For example, the second intentions of 'being a subject' and 'being a predicate' are studied in logic independently of whatever first intentions function as the subject and predicate terms in a given proposition, for example, 'human being' and 'rational animal' in the proposition 'a human being is a rational animal'.