Summarizing chapter 13, Geshe Tashi comments that it is the subjective perspective (ultimate vs. conventional) that establishes the two modes of object existence (ultimately non-existent vs. conventionally existent). We are only capable of grasping the mistaken quality of conventional cognition after having found emptiness with rational analysis. Also, on p.174, Tsongkhapa explains that although ultimate reason refutes intrinsic existence, it does so conventionally, not ultimately. That is because nothing can be done ultimately since nothing exists ultimately. Whatever we do, including rational analysis, must be done conventionally because things only exist conventionally, and only exist contingently.
In chapter 14 LTK takes up the matter of how to distinguish that which exists conventionally from that which does not. He summarizes the problem others have in understanding the distinction like this:
In v.3, p.177, LTK wrote:In this way we Madhyamikas posit conventionally, within our own system, many presentations of cyclic existence and nirvana; we also refute the conventional existence of constructs that are put forward as unique assertions by essentialists. As this is extremely difficult, accurate knowledge of the presentation of the two truths scarcely exists.
Misunderstanding may arise as follows. … If they deny the conventional existence of constructs such as a divine creator or a primal essence, then they must also deny the conventional existence of forms and such; if they hold that forms do exist conventionally, then they would also have to accept the existence of a divine creator. They see those two as equivalent. They say that it is inappropriate for their own system to identify or to assert of any phenomenon, “This is such and such; this is not such and such.” They presume that in this they have found the Madhyamaka reality.
The Three Criteria: LTK asserts that a thing exists conventionally if it is: 1) Known by a conventional consciousness; 2) Not contradicted by conventional valid cognition; and 3) Not contradicted by rational analysis which establishes essential, ultimate existence.
The third criterion is understood this way: if ultimate analysis were to establish that a thing does exist inherently (i.e. independently), it cannot exist conventionally (i.e. dependently), and so its conventional existence would be thereby refuted. But also, certain things, such as intrinsic existence, which are established by the first criteria cannot be disproved by conventional reasoning. So then we apply the third criterion, ultimate analysis, which cannot establish intrinsic existence.
The point is to apply the correct reasoning method for the correct purpose. Conventional reasoning is applied to conventionally appearing objects. Rational analysis is applied to determine the way in which a thing actually exists.
In v.3, p.181, LTK wrote:Since the objects conceived by conceptual consciousnesses that apprehend the aggregates as permanent and so forth do not exist conventionally, reason can refute them. However, the referent objects of the conceptions of the aggregates as impermanent, etc. do exist conventionally; hence, reason cannot refute them. [Emphasis added]
So the objects of mind are of two types: those that can be refuted by reason and those that cannot. Those that can, do not exist; those that cannot, exist conventionally. This fact is critical to understanding Tsongkhapa’s system. He asserts that without understanding how to posit conventionally valid objects, one cannot fully apply the method practices.
This all applies to what LTK calls “mere existence” which is dependent arising with no trace of intrinsic qualities.