I want to say a little more about the Two Truths. In Mahayana Buddhism, the Two Truths are enormously important. Some might say this doctrine is the key the whole shebang, so to speak.
Although the Two Truths are associated with Mahayana, in my research I learned there is support for the Two Truths in the Pali Canon. In the Kaccayanagotta Sutta (Samyutta Nikaya 12.15) the Buddha said,
"By and large, Kaccayana, this world is supported by (takes as its object) a polarity, that of existence and non-existence. But when one sees the origination of the world as it actually is with right discernment, 'non-existence' with reference to the world does not occur to one. When one sees the cessation of the world as it actually is with right discernment, 'existence' with reference to the world does not occur to one."
Now, I know this possibly sounds really, really nerdy to many of you. Well, maybe it is really, really nerdy. But it's a wonderful teaching once you begin to appreciate it. And, as I said, it's key to understanding what Mahayana, the Great Vehicle, is all about. Working with the Two Truths, you begin to see how incessantly your brain tries to turn everything into a dualism.
The Truths tell us that existence can be understood as both ultimate, or absolute; and conventional, or relative. The relative is the world of form and appearance, and the absolute is emptiness, sunyata.
Isn't that a dualism? You might ask. Maybe, but then the Heart Sutra tells us that form is no other than emptiness; emptiness no other than form. The "dualism" is no dualism.
One common mistake people make is to think of the absolute as the "true" reality and the relative or conventional as the "false" reality, but that isn't how it works. These are two truths, not one truth and one lie. Further, the absolute and relative depend on each other. In the Zen text called the Sandokai, we read,
Ordinary life fits the absolute as a box and its lid.
The absolute works together with the relative like two arrows meeting in mid-air.
Sometimes the Two Truths are taught as the Three Truths. Zhiyi (also spelled Chih-i; 538-597), a founder of the Tendai school, taught that the Three Truths are, first, emptiness or insubstantiality; second is relative or contingent; and third is the Middle Way, the inseparability of the absolute and relative. I understand that Nichiren maintained Zhiyi's view in his teaching.
However, on a very basic level I don't think the Two Truths and the Three Truths disagree; it's just a different way of explaining the same thing.
Beyond this very basic understanding of the Two Truths one does find finer points and scholars who disagree about them. But that part of the discussion really is very, very nerdy, so I believe I will stop here.