Answer:
- Reason.
Step-by-step explanation:
'Scientific rationalism' is characterized as the principle that gave chief importance to reason and considered rationality or logic to be the key source in discovering knowledge or justification(as true or false) instead of emotional responses or religious beliefs.
As per the question, Descartes possessed a belief that 'knowledge can rest on a secure structure through reason'. His belief later served to be the basis of movement named 'scientific rationalism' which reemphasized the role of 'reason or logic' in gaining knowledge and reaching valuable as well as trustworthy conclusions. It focused on the use of a 'rational' approach and sources to reach valid solutions instead of adopting an 'emotional or religious' approach. Thus, 'reason' was the key emphasis of scientific rationalism.