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If a security guard sees a guy collapse and he doesn't do anything, which verb should I use to describe his behavior?

He didn't react when he saw the guy collapse.
He didn't act when he saw the guy collapse.

Thanks in advance.
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Without consulting a dictionary to back me up, I'm going to offer that I use 'act' as a physical thing, 'react' more as a mental thing, but not to exclude the physical necessarily. Now that I've confused you, I'm going to say that 'act' is the proper verb here, because he didn't do anything. He probably did have a mental reaction.
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Thanks, Philip. I thinkI get it.