The key to understanding the truth of suffering is what the Buddha called the “three marks” of everything that exists. All conditioned phenomena, he said, are pervaded by these three marks: impermanence (anitya), dissatisfaction or suffering (duhkha), and insubstantiality (anatman, “without self”).
According to the Buddha, if we do not understand how conditioned phenomena are marked by these three aspects, then we will not be able to understand the first Noble Truth. We may do all we can in order to avoid facing the fact that everything is contingent and transient—we may try to hide ourselves from it, and we may even spin out all kinds of metaphysical theories of an unchanging, permanent, substantial reality to avoid this all-pervasive nature of ephemerality. Also, if we do not understand that conditioned phenomena are unsatisfactory, we will not think about restraining ourselves from overindulgence in sensory gratifications, which makes us lose our center and become immersed in worldly concerns, so that our life is governed by greed, craving, and attachment. All of these things disturb the mind.
If we do not understand that everything is insubstantial—anatman—then we may believe that there is some kind of enduring essence or substance in things, or in the personality, and because of this belief we generate delusion and confusion in the mind.
From The Essence of Buddhism: An Introduction to Its Philosophy and Practice by Traleg Kyabgon, pages 4–5