Because of the workings of human language, the number of possible thoughts and sentences is basically infinite.
“Say you have ten choices for the word to begin a sentence, ten choices for the second word, ten choices for the third word, and so on. (Ten is in fact the approximate geometric mean of the number of word choices available at each point in assembling a grammatical and sensible sentence.) A little arithmetic shows that the number of sentences of twenty words of less (not an unusual length) is about 10^20: a one followed by twenty zeros, or a hundred million trillion, or a hundred times the number of seconds since the beginning of the universe.”
This represents the “combinatorial” nature of language, giving rise to and endless stream of possible utterances.
[From Steven Pinker's How the Mind Works]