Phrases such as 'the first thing to observe', where the verb 'observe' could be replaced by other verbs, are frequently seen in English sentences, for example in
The first thing to observe is that both of these equations have a coefficient of 1 in front of the x.
Now, what is meant by that phrase?
The first thing that can be observed
The first thing that should be observed
Does its exact meaning depend on the context in which it appears?