Did you ever notice that a sentence such as "John seeks a unicorn." can have two different meanings:

• John is looking for a specific unicorn which he has in mind.
• John is looking for any unicorn.

If this seems obscure, consider the following example, adapted straight from the Wikipedia article:

Jane wants to marry the richest man in California.

This can mean two different things:

• Jane already knows someone who happens to be the richest man in California, and would like to marry him; we just happen to refer to him as the richest man in California, but that's not its defining quality as far as Jane is concerned.
• Jane doesn't know who the richest man in California is, but would like to marry that man.

This amusing ambiguity is called as the "de re and de dicto" distinction (in the two examples above, the first interpretation is "de re", the second is "de dicto").

Another example of this distinction is Diogenes's "ἄνθρωπον ζητῶ" ("I'm looking for a man"). It is understood as meaning that he would like to find any true man, but, taken out of context, it could also mean that he has a specific man in mind and is looking for him.

For French speakers, it is interesting to compare "Je cherche un homme qui a les yeux bleus." (indicative) to "Je cherche un homme qui ait les yeux bleus." (subjunctive). The first sentence is de re (I'm looking for someone specific who happens to have blue eyes) and the second is de dicto (I'm looking for anybody with blue eyes).