||Philosophers and linguists reflect in a variety of ways on the natures of words. One range of issues here concern the metaphysics of words: are words concrete items in the world, kinds of items, or elements of some other category? What are the principles for counting words? Are there ambiguous words, or are there, for example, a variety of words each spelled "bank"? Are words basic, or are they built from more basic elements, like morphemes, features, or letters? Connected with the last question, philosophers and linguists have discussed issues about the internal semantic structure of words, a version of the question whether words are definable. Sometimes this issue is pursued via the question, are there building blocks for words that can only be combined in a limited range of ways and thus make it impossible for there to be certain words, at least in normal human languages?