Natural science is the science of nature.

But--What is nature?
Nature is that which we observe in perception through the senses.

In this sense-perception we are aware of something which is not thought and which is self-contained for thought.

This property of being self-contained for thought lies at the base of natural science.

It means that nature can be thought of as a closed system whose mutual relations do not require the expression of the fact that they are thought about.
Thus in a sense nature is independent of thought.