‘Discipline’ as used in the above passage means

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The learning of a foreign language should be an integral part of every university student’s education. As a discipline, it affords the mind excellent training. It utilizes skills needed in other subjects; the concentration required for memorizing music (not to mention for developing the ear for tone and pitch), the logical thinking for solving mathematical problems, even the deductive reasoning capabilities needed to comprehend philosophical concepts. The mind is stretched through the simple act of learning the grammatical patterns, vocabulary and phonological system of a language. At the same time, the student of a foreign language is acquiring a useful tool. This tool has a double advantage, for while most people (especially teachers) expound on the reward for being able to communicate in a second, third, or fourth language. Perhaps the more promising of the two, is being able to understand and hopefully appreciate another cultural group through their literature, their music and custom – all of which are best assimilated through the medium of their language.