Theoretical investigation, when pursued in order to gain knowledge, is directed to one of two ends: either to conception or to assent. What leads to conception is called an “explanatory statement”; under it fall the definition (ḥadd) and the description (rasm). What leads to assent is called a “proof,” which may be a syllogism (qiyās), an induction (istiqrāʾ), or something else.
Al-Ghazālī said:
“The substance of this book is to set out the principles of the explanatory statement by which the sought-for conception is obtained, whether by definition or by description, and to set out the principles of the proof that brings about assent, whether by syllogism or otherwise, while drawing attention to the conditions of their soundness and the sources of error within them.”
He went on to say:
“If you ask, ‘How can a person be ignorant of conceptual knowledge so that he needs a definition?’ we reply: It happens when a person hears a name whose meaning he does not know—like someone who asks, ‘What is the khalaʾ (vacuum)? What is the malaʾ (plenum)? What is Satan? What is ʿaqār?’ He is told: ʿAqār is wine. If he does not recognize it by that familiar name, he is made to understand it through its definition: Wine is an intoxicating drink pressed from grapes. In this way he acquires conceptual knowledge of the very essence of wine.”
I say: He has thus shown that the benefit of definitions is of the same kind as the benefit of names. It is as though one hears a name whose meaning he does not know; another name is then offered, and if he still fails to picture the object, a definition is provided. This is acknowledged by their most astute scholars; even their “second teacher,” Abū Naṣr al-Fārābī—who was the most expansive of the philosophers in matters of logic, its branches, and its demonstration—made this clear. It is well known that a name, by itself, does not present an image of the thing named; it merely distinguishes it from other things. As for conceiving the object itself, sometimes a person conceives it directly by inner or outer perception, and sometimes he conceives it by picturing something analogous to it, which is a more remote way. Thus, whoever knows the very entity