The word ‘post’ as a vertical pole appears in a number of standard expressions in English: lamp post, fence post, hitching post, goal post. Hitching posts for horses are pretty rare nowadays, but at one time they were quite common. Although fence posts are still common physical artifacts, the phrase has acquired a fairly recent metaphoric extension in the world of computer programming — a ‘fence post error’. The beginning and end of an indexing routine or iteration can be thought of figuratively as ‘fence posts’, analogous to placing a physical post at the beginning and end of a stretch of fence that is being built. In the physical world, if you intend to place a fence post for every ten feet of horizontal fence being built, and place one of the posts nine or eleven feet from the previous one, you’ve committed a fence post ‘error’. In a computer program, if you write a routine to copy all the characters of a word to another location, you can access the characters of the word by their position in the string, i.e, by their index. If your program assumes that the first location has index value ‘0’, then a five-letter word will be copied after you have accessed index position ‘4’ (0,1,2,3,4). If your program assumes the first location has the index value ‘1’, a five-letter word will be copied after you have accessed index position ‘5’ (1,2,3,4,5). Thus, in the case where indexing starts at ‘0’ and you iterate through ‘5’ for a five-letter word, a ‘fence post error’ has been committed — the endpoint is now beyond where it should properly be.
Goal post has also taken on a metaphoric meaning in recent times, as when people accuse their adversaries of ‘changing the goal posts’ in the context of debates on abstract topics, especially in regard to policies and agendas related to politics or business.
It’s a nifty cognitive strategy to take a concrete idea and employ it as a shorthand ‘hook’ for an abstract complex of activity (iterative indexing) or communication (debating tactics). We shouldn’t forget ‘signpost’ whose basic meaning refers to information displayed on a board held up on a pole — the metaphoric extension here is pretty obvious, and also quite broad — it refers to anything that provides guidance or clues with respect to unclear and complicated situations.
It would be fun to investigate figurative usage across English (or other languages) to see whether and what patterns emerge, e.g., which metaphoric extensions are narrow in scope and which are broad, and what attributes of the basic, concrete sense of the word may influence the metaphoric utility of its meaning. I’d also like to figure out a way to measure the rate of increasing (or decreasing) abstractness of a language. As new meanings enter English, for example, and old ones fall out of usage, is the language retaining a fairly constant ratio of concrete/abstract senses, or is the language becoming increasingly abstract? Or increasingly concrete? An objective and quantifiable notion of what is meant by ‘abstract meaning’ in the lexicon would be required to track this, and such a metric may be difficult to define with precision and agreement across speakers of the language. Something worth pondering, though.