With Mercury in Sagittarius the logical mind is under the influence of a fire sign. It may become quickly and passionately interested in a cause. The sign of Sagttarius influences the thinking to be directed towards matters of concern for society as a whole. Ideas of Religion and Philosophy may well be of interest. Also ideas of foreign lands and cultures and world travel may inspire the individual. Mercury is in detriment in this sign since it opposes one of the signis it rules which is Gemini. This may well be because the broad societal values which are of interest to the sign of Sagittarius can lose sight of the detailed logical facts which is what the planet Mercury represents.