Question

One scientific belief held by both René Descartes

Answer 1

Answer;
One scientific belief held by both Rene Descartes and Isaac Newton is that the reasoned thought is the way to discover truth. 

Explanation; 
Rene Descartes used deductive reasoning, deducing a conclusion from a set of premises, to determine the nature of the universe, that was contrary to other great thinkers of the time who used scientific observation.
Sir Isaac Newton; specialized in uniting experimental and theoretical elements of modern science. 

Answer 2

One scientific belief held by both René Descartes and Isaac Newton is that "(1) reasoned thought is the way to discover truth," since both of these men were progressive thinkers who challenged the religious status quo.