Given the following clause, think of a relevant word from your experience so that the following clause can acquire more depth.
He was a keen _________ .
He was a keen observer.
Complete the following task based on the instruction and optional input provided.### Instruction:
Given the following clause, think of a relevant word from your experience so that the following clause can acquire more depth.

### Input:
He was a keen _________ .

### Output:
He was a keen observer.