The Trilogy of Anicca, Dukkha and Anatta - By Bhikkhu Bodhi

The Buddha says that we have to examine our experience in order to discover its most pervasive features, the universal characteristics of phenomena, namely, impermanence, unsatisfactoriness and egolessness or notself.
The Buddha says:

All formations are impermanent.
All formations are unsatisfactory.
All phenomena, everything whatsoever, are not self.

Formations are things which arise from causes and conditions. They include all compounded or formed phenomena. Although all formations around us have these three characteristics, we are unable to see them because our minds are ordinarily cloaked by ignorance. Ignorance is a mental factor which has been covering the minds of all sentient beings through beginningless time. It covers the minds of every one but the fully enlightened ones, the Buddhas and the arahants.

Ignorance functions in two ways, negative and positive. On the negative side it simply obstructs us from seeing things as they are; it throws up clouds of mental darkness. On the positive side, it creates in the mind illusions called perversions. Due to these perversions, we see things in quite the opposite way from the way they really are.

These perversions are:
(a) Perversion of seeing what is unattractive as attractive.
(b) Perversion of seeing what is Dukkha or unsatisfactory as pleasurable.
(c) Perversion of seeing what is impermanent as permanent.
(d) Perversion of seeing what is really not self as self.

These illusions give rise to craving, conceit, wrong view and all other defilements, and in that way we become entangled in dukkha.

These universal characteristics have to be understood in two stages: first intellectually, by reflection; and thereafter by direct insight or realisation through insight meditation. When we explain these intellectually, we should not make this a substitute for practice, but only take it as a guideline for understanding what has to be seen by the actual practice of insight meditation.