Buddhism began not with a doctrine but with an experience. Siddhartha Gautama saw suffering — and went away to understand it. What he found overturned the thinking of two continents. This route runs from the first enlightenment to Nagarjuna's philosophy of emptiness, and on to how Schopenhauer opened up the Buddha for Europe — half a century before it became fashionable.
Gautama is the starting point. The Four Noble Truths: life is suffering, dukkha; suffering has a cause; the cause is craving; there is a path to the ending of craving. This is not pessimism — it is a diagnosis and a cure.
Nagarjuna created Madhyamaka — the philosophy of the middle way between being and non-being. His central notion: śūnyatā, emptiness. No thing exists in and of itself — only in dependence on others. This is more radical than it sounds: what is called into question is the “I” itself.
Schopenhauer was no Buddhist — but his “The World as Will and Representation” reached similar conclusions by another road: behind suffering stands a blind will to life, and release lies in the denial of that will. He was the first in Western philosophy to set Buddhism on a level with Kant.
If the “I” is an illusion, as Buddhists claim, then who is it that practises meditation and strives for enlightenment?