In the Southern Buddhist tradition, the Theravada, the two levels of truth are often spoken of. In Pali, “conventional truth” is known as sammutisacca or vohārasacca, conventional reality. And then, the “ultimate truth” is called paramatthasacca, the ultimate reality.
These are described in various different ways, but one illustration that I feel is very effective and helps us to understand is to say that, conventionally speaking, the sun rises in the east in the morning and sets in the west in the evening. That's the conventional reality.
However, ultimately, the sun doesn't rise or set. Rather, it's the spinning of the earth, the turning of the earth, that gives the appearance of a rising and setting. That gives us a very clear picture of something that seems to be true – an ordinary, everyday, natural reality: sunrise in the morning, sunset in the evening. But, in actuality, when the position or the point of view is changed, for instance leaving the surface of the earth and going into space, the sun is not moving in that same way of rising and setting, but rather it's simply the point of view that has caused that appearance.
All analogies are only relatively applicable, but even though this might have some shortcomings, in that the sun is actually moving, it's expanding with the rest of the galaxy, it's spinning around and has its own movement, yet according to the needs of this particular analogy, we can say that the sun is still and it appears to rise and set according to the turning of the Earth. So, it's a good way of illustrating sammutisacca and paramatthasacca and how reality can be viewed in different ways.
Venerable Ajahn Chah spoke of the importance of understanding the difference between conventional truth and ultimate truth, sammutisacca and paramatthasacca, or what he called vimutti, because of the way they alliterate in Thai, “kong samut and kong vimut,” in a particular passage at the beginning of his Dhamma talk known as “Convention and Liberation,” which I'll read here:
The things of this world are merely conventions of our own making. Having established them, we get lost in them and refuse to let go, giving rise to clinging to personal views and opinions. This clinging never ends. It is samsara, flowing endlessly on. It has no completion. Now, if we know conventional reality, then we'll know liberation. If we clearly know liberation, then we'll know convention. This is to know the Dhamma. Here there is completion.
There are numerous places in the Pali Canon, the discourses of the Southern Buddhist tradition, where the nature of conventional reality is illustrated or is talked about. For example, it is spoken of in the Bhikkhuni Vagga of the Connected Discourses, where the nature of a chariot is described as a way of illustrating the empty nature of the human body.
We can say that a chariot is made up of wood and metal and fittings made of leather and so forth. And then you put the wheels and the footplate and the axle together, and you connect it all up and you say it's a chariot. But each individual part has no intrinsic “chariot-ness” to it.
Similarly, of a person, you can say there are bones and blood and a brain and thoughts and feelings, but each of the individual elements has no absolute personhood to it. Put it together and we say “person” as a convention, but ultimately, there is no person absolutely and completely independently there.
So, that is a very helpful teaching, and you can find that in the Connected Discourses, the Saṃyutta Nikāya.
In Sutta number ten in the Bhikkhunī Saṃyutta, concerning the Bhikkhunī Vajira, it says:
Then, Mara the evil one, desiring to arouse fear, trepidation and terror in the Bhikkhunī Vajira, desiring to make her fall away from concentration, approached her and addressed her in verse, “By whom has this being been created? Where is the maker of the being? Where has the being arisen? Where does the being cease?”
Then it occurred to the Bhikkhunī Vajira, “Now, who is this that recited the verse? A human being or a non-human being?” Then it occurred to her, this is Mara, the evil one, who has recited a verse desiring to arouse fear, trepidation and terror in me, desiring to make me fall away from concentration.
Then the Bhikkhunī Vajira, having understood that this is Mara, the evil one, replied to him in verse, “Why now do you assume a being, Mara? Is that your speculative view? This is a heap of sheer formations. Here, no being is found. Just as, with an assemblage of parts, the word chariot is used. So, when the aggregates exist, there is the convention, a being. It's only suffering that comes to be. Suffering that stands and falls away. Nothing but suffering comes to be. Nothing but suffering ceases.”
Then Mara, the evil one, realizing that Bhikkhunī Vajira knows me, sad and disappointed, disappeared right there.
This is a theme that's frequently addressed by many of the forest Ajahns of our tradition, of old and into the present day. Of course, this is naturally a repetitive theme because it's so liberating and so helpful for changing the view from one that's self-based, an ego-centered perspective, to a nature-centered or dhamma-centered perspective.
Venerable Ajahn Chah himself would often say, from the dhamma seat when giving a talk:
There are no women here, there are no men here, there are no nuns, no monks, no lay people, no monastics. These are merely conventions, merely forms. They come into being and we label them monk, nun, layperson, monastic, old, young, tall, short, Thai, British, American. These are merely conventions. There’s nothing absolute and substantial there.
That is a frequent teaching, not just given to monastics, but also to laypeople as a way of helping to free the heart from ego-centered views and be more in tune and awakened to their own ultimate reality. I hope that these few words on conventional truth and ultimate truth are helpful.