Scientist Charles Darwin is renowned, of course, for his theory of evolution by natural selection.

During the 1830s, he went on a five-year voyage on the HMS Beagle, and during that time, he kept extensive notebooks detailing his observations.

One of these notebooks, now called Notebook B, was written in 1837 and 1838. On one page, he drew a very rough tree, with notes that reveal that he was groping toward the ideas of evolution.

What was striking to me was that at the top of the page, above the sketch of the tree shape and his notes to himself, he wrote, “I think.”

I can’t stop thinking about this image, and I was trying to figure out why.

I think it’s because to me, this phrase could be read three ways, each of which is very useful to anyone who is trying to do original thinking.

The first reading is that he’s reminding himself, “I think.” “I am a thinking being, I am observing, I am testing theories, I am pulling together my thoughts.”

The second reading adds an element of caution, a reminder to stay open to criticism or opposing arguments. “I’m not sure, but I think…”

The third reading is that those words serve as a kind of header, as if Darwin has posed the question to himself, “What do I think?” and answered himself by writing “I think” followed by a summary of his current views. To me, this reading rings the most true, but to be sure, this is the kind of thing I do myself.

All three readings are worth remembering.

Here is that famous page.