The saying that someone "can't see the forest for the trees" means that he is so involved with the details of a situation that he loses sight of the larger issue. It is a fairly common expression in English, though the use of "for" can be confusing for some people, since it is a more archaic meaning in this idiom. This expression can also be reversed, indicating that a person loses sight of details and becomes engrossed in the whole.
Someone who can't see the forest for the trees has typically become so focused on details that he or she begins to ignore the overall situation. People might also phrase this expression as "you can't see the wood for the trees," which is the more common form in the UK. A person accused of being unable to see the forest may want to take a step back from the situation, to regain a wider perspective on a problem.
It is very easy to get caught up in minutia of a situation, especially when someone works on a problem for an extended period of time, or has only been working on one aspect of a larger issue. Being aware of this tendency can make people better problem solvers, as they know that it's a good idea to occasionally talk with other people about the aspects of the project that they are working on. Such outside advice about a situation can help a person gain perspective and approach a problem from a new angle.
History of the Expression
As early as the 1500s, "you can't see the forest for the trees" was in wide enough use that it was published in collections of proverbs and slang. As anyone who has been in a forest knows, it can be easy to fall into the trap of just looking at the individual trees, rather than considering the forest as a whole. Some confusion can arise over the use of "wood" rather than "forest," because it may imply a person is focused on the wood of the trees rather than the objects themselves. In this usage, however, "wood" refers to a small forest, rather than the substance of which trees consist.
Reversal and Other Meanings
This proverb is also sometimes reversed, as in "you can't see the trees for the forest," referencing the idea that it is also possible to be too broad when looking at a situation. Someone who makes sweeping pronouncements without considering various details could exhibit just as much of a logical flaw as someone who only focuses on the details. It is common for executives to be accused of not seeing the trees for the forest, especially when they make exacting and impossible demands that suggest a complete unfamiliarity with the complexities of a project.