The Two Truths – an important Buddhist doctrine or belief.
One might quickly call them appearance and reality, but that would be misleading (>>>). Thich Nhat Hanh condenses the idea to relative and absolute truth. He likes the metaphor of the wave and the ocean, where the wave has its good moments and bad, but we know it will dissolve back into the ocean with a little time.
I find the more technically philosophical level of explanation given by Jay Garfield in his commentary on (and translation of) Nagarjuna’s Fundamental Wisdom of the Middle Way right at the outer edge of my mental capacity. I have parked the opening of his commentary here.
Of the many presentations of the idea (of emptiness or sunyata) you can find on the Internet, this one (written by William Herbrechtsmeier, Professor of Religion at Humboldt State University) appears to fit with Garfield’s take on Nagarjuna and is easier for me to understand.
It looks to me like the Western world of philosophy has centered most of its thinking energy around the paradoxes that arise in attempting to distinguish clearly between the inner life of the mind and the outer being of things as they “really” are. But Buddhism seems to be quite definitely of the opinion that they “really aren’t.” That would be the doctrine of emptiness.
Follow the links on the left to find out more. Follow this Encylopedia Britannica link for more on the Western struggle with realism. Or try this pdf I made of two pages on the topic in W. L. Reese’s Dictionary of Philosophy and Religion (1980).