Philosophical speculations are of secondary importance and, anyway, they are best left until after one has well trained the mind in meditation to the stage where one has the ability to examine the matter clearly and find the Truth for oneself.
Thus, the central Teaching of the Buddha, around which all other teachings revolve is the Four Noble Truths:
1. First Noble Truth: Life is subject to 'suffering'It would be mistaken to label this Teaching as 'pessimistic' on the grounds that it begins by centring on suffering. Rather, Buddhism is 'realistic' in that it unflinchingly faces up to the truth of life's many sufferings and it is 'optimistic' in that it shows a final end of the problem of suffering - Nibbana, Enlightenment in this very life! Those who have achieved this ultimate peace are the inspiring examples who demonstrate once and for all that Buddhism is far from pessimistic, but it is a Path to true Happiness.
2. Second Noble Truth is that this suffering is caused by ignorance which results in desire
3. The Third Noble Truth is that suffering can be eliminated by the removal of desire
4. The Fourth Nobel Truth is the path to eliminate desire.