Another question connected with the nature of the essence is, whether it is caused or uncaused. The followers of Aristotle, or philosophers as they are generally called by their opponents, hold that the underlying essence of things is uncaused. The As̱ẖ‘arite hold the opposite view. Essence, says the Aristotelian, cannot be acted upon by any external agent.[90:1] Al-Kātibī argues that if, for instance, the essence of humanity had resulted from the operation of an external activity, doubt as to its being the real essence of humanity would have been possible. As a matter of fact we never entertain such a doubt; it follows, therefore, that the essence is not due to the activity of an agency external to itself. The idealist starts with the realist distinction of essence and existence, and argues that the realist line of argument would lead to the absurd proposition--that man is uncaused; since he must be regarded, according to the realist, as a combination of two uncaused essences--existence and humanity. [90:1] Ibn Mubārak's Commentary, fol. 20a. B. _The Nature of Knowledge._ The followers of Aristotle, true to their position as to the independent objective reality of the essence, define knowledge as "receiving images of external things".[91:1] It is possible to conceive, they argue, an object which is externally unreal, and to which other qualities can be attributed. But when we attribute to it the quality of existence, actual existence is necessitated; since the affirmation of the quality of a thing is a part of the affirmation of that thing. If, therefore, the predication of existence does not necessitate actual objective existence of the thing, we are driven to deny externality altogether, and to hold that the thing exists in the mind as a mere idea. But the affirmation of a thing, says Ibn Mubārak, constitutes the very existence of the thing. The idealist makes no such distinction as affirmation and existence. To infer from the above argument that the thing must be regarded as existing in the mind, is unjustifiable. "Ideal" existence follows only from the denial of externality which the As̱ẖ‘arite do not deny; since they hold that knowledge is a relation between the knower and the known which is known as external. Al-Kātibī's proposition that if the thing does not exist as external existence, it must exist as ideal or mental existence, is self-contradictory; since, on his principles, everything that exists in idea exists in externality.[92:1] [91:1] Ibn Mubārak, fol. 11a. [92:1] Ibn Mubārak, fol. 11b. C. _The Nature of Non-existence._