"Well it's something which is really quite simple."
Sangarakshita in Seminar
"Well it's something which is really quite simple. If one takes it by easy stages; first of all you just see that everything changes. You could say, if one wanted to reduce it to its simplest terms, that this is the basic, the fundamental insight of Buddhism itself, or the Buddha himself, that everything changes, that nothing lasts, that nothing remains the same for two consecutive instants. This is the basic insight, it is really as simple as that.
It is said that there are three great characteristics of mundane existence, that it is impermanent, painful, and devoid of permanent unchanging self. But you can deduce these other two characteristics from this first one, because, if everything is constantly changing, then it means you cannot retain your hold on anything, because things change at the very moment that you’re holding on to them.
You can't retain anything for good, but if you try to do that you are going against the very current of existence. So if you will insist on holding on to something which by its very nature, by virtually the very nature of existence itself, you cannot possibly hold on to. If there is that sort of tension, if you are pulling in one direction, the whole of the rest of existence is pulling in the other, what's going to be the result?
Suffering. You are pitting your sort of puny will against the will so to speak of the whole universe, so it's obvious who's going to get the worst of the encounter. So that is suffering. Your refusal not only to recognize but to act upon the truth of suffering, and the truth of impermanence, that is how suffering arises, because if you were to recognize the truth of suffering and act upon it all the time you would never experience suffering, certainly not mental suffering. You might experience physical suffering, that would be inseparable from embodied existence itself, but you would never experience mental suffering, that would be impossible, in fact you would be enlightened. Just by realising the truth of and acting upon that simple teaching of impermanence."
the above excerpt has been edited slightly. Please click on this link to the 1982 seminar transcript to see the complete original text which is available on the Free Buddhist Audio website