The answer is Psychological Processes.
To complete the sentence (second):
This led him to suspect that these symptoms resulted from psychological processes.
Quick information:
Psychological processes are used during acquisition. The most basic psychological processes are sensation, perception, attention, learning, and memory.
You need a variable. If something happens once it could just be chance. Like if you got a lottery ticket and won, it would just be chance. Not the thing that happens the most.
