The reasons why solitude is to be sought for the time of this practice are several. Here are two. First, he can give greater attention to it than when the presence of others draws thoughts to them. Second, there is a psychic aura which pervades the body and spreads outside it. If he is near enough to come in contact with it, he may be afflicted as by a contagion. Alien thoughts will then intrude upon his mind and hinder the meditation.
-- Notebooks Category 4: Elementary Meditation > Chapter 2: Place and Condition > # 139