“The thinker has to decide, to choose; and there is always a risk.”
Choosing among plausible interpretations necessarily involves uncertainty, so sound thinking requires a cautious selection that remains open to confirmation or correction by later events.
Source note: How We Think, Chapter VIII. The sentence concerns choosing the idea or principle relevant to a perplexing situation; the next clause says the prudent thinker selects warily, subject to later confirmation or frustration.