The in debt and doubt was inserted to reflect Latin cognates like debit and dubitable. Similarly with alien clusters such as Greek initial in psychology and in mnemonic.
Similarly with /, the latter from Greek phi. The spelling was in Old English pronounced /x/ in such words as light. The bad news is that English has a lot of silent letters, and they create problems for both native and non-native speakers of English, because they make it more difficult to guess the spelling of many spoken words or the pronunciation of many written words. A silent letter is a letter that appears in a particular word, but does not correspond to any sound in the word's pronunciation.
