(9.138) (Other schools) “According to you Madhyamikas, valid cognizers are not truly existent, and so they must be false,
In which case the objects established by them must also be false.
If this is so, then the emptiness you assert is false,
And meditating on it serves no purpose.”

This is a very clever argument by the other schools, and something that we ourselves may generate doubts about. The Prasangikas say that conventional truths are mistaken appearances. We then grasp at these mistaken appearances, assenting to them as existing in the way that they appear, and therefore generating mistaken conceptions. If this is the case, then how is it possible for a non-Buddha to have any valid cognizers at all if all of the conventional objects realized by that mind are mistaken? If they cannot have any valid cognizers, then how can they ever generate any realizations? If the conventional truths that they meditate on are mistaken, how can they produce an unmistaken result of enlightenment? Only something true can create a true effect, and a mistaken cause can never produce an unmistaken effect.

(9.139) Without first correctly identifying the object to be negated, true existence,
You cannot apprehend its negation, or non-existence, emptiness.
The negation of true existence, emptiness,
Clearly has no true existence itself.

(9.140) For example, if a mother dreams her child has died,
The thought that the child no longer exists
Removes the thought of the child’s existence,
Even though neither thought is truly existent.

The Prasangika’s answer to this objection is quite helpful in not only answering the doubt, but showing how a being can go from being in samsara to attaining a state beyond samsara.

As explained before, emptiness is the mere lack of inherent existence. It is a non-affirming negative phenomena, which means it is realized by negating something specific, namely inherent existence. Practically speaking, when we meditate on emptiness, we first identify the object of negation within our own mind and realize how we are grasping at it, and then we apply the reasonings provided by the instructions to dismantle this wrong view. If we do not correctly identify the object of negation, it is impossible to find the emptiness or lack of that object.  We have already established previously that there is no enlightenment outside of realizing the emptiness of true existence. So we first have to identify a mistaken view clearly within our mind, and then apply valid reasons to dismantle that view to produce the effect of a realization of emptiness within our mind. We do not say that the mistaken appearance and the mistaken conceptions are causes of enlightenment, they are the objects to be negated by the valid reasons establishing emptiness.

Prasangikas agree that only unmistaken causes can create an unmistaken effect. Identifying inherent existence within our mind is a precondition for applying the valid reasons establishing emptiness. It is the valid reasons establishing emptiness that take us to enlightenment, not grasping at the true existence of the object of negation or the mistaken appearance.

A doubt may arise but aren’t our reasons establishing emptiness also conventional truths, and therefore likewise mistaken?  Here, we need to understand that there are many different levels and degrees of mistaken appearance and conception. For example, unicorns and mermaids can be conceived by mind as being truly existent. Indeed, there are many people who believe in such things even in this world. Conventionally, though, such beings have never been found. If we looked around the world, we would never find a unicorn or a mermaid.  Unicorns and mermaids do not exist conventionally.  They also do not exist ultimately in the sense of being truly existent, even though there are many people who believe that they do. Thus, unicorns and mermaids are mistaken both with respect to conventional truth and ultimate truth. In contrast, horses and dolphins do exist conventionally, but they do not truly exist ultimately.  In this sense, unicorns, mermaids, horses, and dolphins are all equally non-existent ultimately, but horses and dolphins do exist conventionally. This shows there are different degrees of mistaken awareness and conception.

Using this analogy, we can understand that the valid reasons establishing emptiness can also have different degrees of mistaken-ness within our mind. The generic image of the valid reason establishing emptiness is conventionally more true than the valid reasons establishing the object of negation. The object of negation is like a unicorn or a mermaid, Whereas the valid reasons establishing emptiness are like horses and dolphins. They do exist and function conventionally, even though all of these things do not exist truly in the sense of being inherently existent.

And even among conventional valid reasons establishing emptiness, there can be different degrees to which we understand the reasons themselves are also empty of inherent existence. If we have a partial understanding of the emptiness of the valid reasons establishing emptiness, then those reasons are conventionally speaking more valid than a valid reason establishing emptiness that we think truly exists. So first we apply conventionally valid reasons, then we later realize the emptiness of these valid reasons, and are gradually lead to a state in which we realize the emptiness of everything, including the valid reasons establishing emptiness.

This is not that difficult to understand. For example, if we had a very dirty countertop and a very dirty sponge, we might not be able to clean much at all. But if we rinse out the sponge partially, it will be able to soak up some of the grime on the countertop, making it more clean. If we rinse out the sponge completely, then we can soak up even more grime on the countertop. The very dirty sponge is like trying to use invalid reasons to establish a valid conclusion. That is impossible. The partially clean sponge is like using a valid reason to establish a valid conclusion. The completely clean sponge is like using a perfectly pure reason to establish a yoga direct perceiver, which is even deeper than valid cognizers are.