From The Complete Works of Swami Vivekananda Volume 7- Chapter 4 – Notes Of Class Talks and Lectures
The Mantra – shastris (upholders of the Mantra theory) believe that some words have been handed down through a succession of teachers and disciples, and the mere utterance of them will lead to some form of realisation. There are two different meanings of the word Mantra – chaitanya. According to some, if you practise the repetition of a certain Mantra, you will see the Ishta – devata who is the object or deity of that Mantra. But according to others, the word means that if you practise the repetition of a certain Mantra received from a Guru not competent, you will have to perform certain ceremonials by which that Mantra will become Chetana or living, and then its repetition will be successful. Different Mantras, when they are thus “living”, show different signs, but the general sign is that one will be able to repeat it for a long time without feeling any strain and that his mind will very soon be concentrated. This is about the Tantrika Mantras.
From the time of the Vedas, two different opinions have been held about Mantras. Yaska and others say that the Vedas have meanings, but the ancient Mantra – shastris say that they have no meaning, and that their use consists only in uttering them in connection with certain sacrifices, when they will surely produce effect in the form of various material enjoyments or spiritual knowledge. The latter arises from the utterance of the Upanishads.