How do you know the natural world is real?
The Buddha’s provisional worldview could not be purely materialistic. He established this point with the line that his followers posted in the first line of the Dhammapada: “The heart/mind is the forerunner of all phenomena.” With this line, the Buddha rejected the worldview in which the mind is simply the passive recipient of sense data, or in which its functions are nothing more than the after-effects of physical processes.
In order to end suffering, a provisional worldview was adopted by the Buddha. This article discusses that view and the strategic reasons for adopting it.