Thinking is the hardest work there is, which is the probable reason why so few engage in it.

The sentence frames “thinking” as labor—strenuous, demanding, and therefore avoided—rather than as a passive or effortless activity. By calling it “the hardest work,” the quote challenges the tendency to equate work solely with physical exertion or routine tasks, and it criticizes intellectual laziness and conformity. The second clause (“probable reason why so few engage in it”) adds a sardonic social observation: many people prefer habit, received opinion, or mechanical busyness to the discomfort of analysis and independent judgment. In a Fordian register, it also implies that progress—personal or industrial—depends on sustained, disciplined reflection.

1) “Thinking is the hardest work there is. That is probably the reason why so few people engage in it.”
2) “Thinking is the hardest work there is, which is probably the reason so few people do it.”