The Samkhyas say that effects are the nature of their causes, but that both are still somehow inherently existent. We saw this with the example of the acorn and the Oak tree.  Now Shantideva refutes this possibility.

(9.135) Although you do not want to assert that a manifest phenomenon that did not previously exist is produced anew,
In reality this is what you are saying.
If the effect exists as the same nature as its cause,
Then eating food is the same as eating the excrement it produces!

(9.136) So instead of spending your money on clothes of woven cotton,
You might as well purchase cotton seeds to wear!

If the cause still exists at the time of the effect then we would have to say that the causes and their effects are in fact one in the same thing, at which point food is excrement. For something to come into existence, the effect, the cause needs to cease completely. Cause and effect necessarily implies a relationship in time, where cause proceeds effect. How can something be a cause of something if the effect already exists? That would imply that the cause is not necessary and the effect already exists and therefore there is no difference between cause and effect. They are one in the same thing. If the effect already exists at the time of the cause, then what need is there for the cause itself? And can we say one thing caused another if that other thing already exists at the time of the cause?

The acorn ceases entirely at the time of the Oak tree. If we look at the Oak tree, we cannot find the acorn that gave rise to it anywhere.

(Samkhya) “Worldly people do not see the effect at the time of the cause because of their confusion.”

The Samkhyas here are saying that worldly people only see the present and not the inevitable effect in the future and so therefore do not realize that the effect and the cause are both existing at the same time.  Again, this seems quite a reasonable view.

Well, what about your teacher, Kapila? He must know because you say he is omniscient;

(9.137) And since you teach his view to worldly people,
Why can they not see the effect at the time of the cause?
(Samkhya) “Because worldly people do not see things with valid cognizers.”
Then the manifest phenomena that they see clearly must also not be true!

The essential point here is the Samkhyas assert that normally living beings are seeing things correctly. The definition of truly existent is things exist in the way that they appear. Things appear to exist from their own side, independent of our mind, on the side of the object. The Samkhyas agree with this and say things do indeed exist in this way. They are seeing things truly. But here, Shantideva points out a contradiction. If living beings have valid cognizers about how things exist, then how can you say worldly beings do not see things with valid cognizers when it comes to cause and effect? Either they are seeing things correctly or they are not. If their view is mistaken, which the Prasangikas say it is, then things are not truly existent. If their view is correct, which the Samkhyas say it is, then they should be able to see effects existing at the time of the cause (if such a thing existed).

This discussion reveals the emptiness of time, which is explain in more detail in Ocean of Nectar. But for here, there are three main reflections that flow from these teachings that establish the emptiness of time.

First, time is established as conventionally existent through the relationship between cause and effect – cause comes before effect, and effect comes after cause. So a relationship in time is established as dependently existent.

Second, the Samkhyas are close when they say that the effect exists at the same time as the cause, but not quite correct. In the present moment, when the cause exists, the inevitable effect exists as a future thing. It does not exist at the same time as the cause, but the “future of the thing” exists at the time of its cause. In other words, the oak tree exists as a future effect in the present moment. When we think about our future, we are doing so in the present moment. The future we imagine is an object of the present that exists at the same time as the present, but its mode of existence is as a “future thing.” The future itself does not exist in the present moment, but the “future effect” exists as an imagined idea in the present. Likewise, our past exists in the present moment as a “past event.” Even though the past itself has completely ceased, in the present moment we have a memory of our past. The memory of our past exists in the present moment. So we can’t say that the past exists in the present, but we can say that the memory of our past exists in the present moment, and other than this memory, there is no past at all remaining. There is no past out there still existing, it has ceased completely, but within the present moment the “past of a thing” exists. So in the present moment we have three things – “the thing,” “the past of a thing,” and “the future of a thing.” We do not, however, have the past, present, and future all existing at the same time as the Samkhyas are asserting.

Third, when we examine the nature of both memories and future imaginings, they are quite clearly both mere mental projections of mind. Besides these mental projections, there is no past and no future at all – there is only the present moment. So we can see clearly how both past and future are mere projections of mind. Since the present moment doesn’t abide even for an instant, there is no inherently existent present either. The present moment is like the aggregate of the same unfolding process of transformation, seen as cessation from one angle and production from another angle, but in fact it is the same thing – just looked at from two different angles. So within the “present” we have the “past of things,” the “future of things,” “cessation,” and “production.”

Further, both our memories and our future imaginings are not fixed, but can be reimagined. For example, all of us have experience of having had some trauma in the past but when we look back on it now, we see it as the best thing that ever happened to us. I had a boss once who went blind when he was in grad school. He told me, “I thank God every day for having made me blind, because without that, I wouldn’t be the person I am today.” He went on to say, “and since we know this is possible, the secret to life is to live our life from this perspective, knowing we will look back on our present troubles and say it was exactly what we needed to become the person we have become, so why not view it that way now?” Such wisdom! Similarly, when we think about the future, we can imagine and reimagine our future in countless different ways. Indeed, this is the essence of the entire Tantric path. Our spiritual guide has presented us with a vision of who we will become – a Heruka or Vajrayogini. We never even knew such a thing existed, much less imagined it as our future. But now we can, and indeed he invites us to bring that future result into the present. Which we can do because time is empty, the future is empty, the present is empty.

Understanding the emptiness of time, which flows from this seemingly esoteric debate with the Samkhyas about effects existing at the time of their cause, is one of the most liberating emptinesses to realize. As Nagarjuna said, “when emptiness is possible, everything is possible.”