It is in this article, my dear Philothea, that I wish you with special affection to follow my counsel, for in it lies one of the most assured means of your spiritual advancement. As often as you can in the course of the day recall your spirit into the presence of God [...]
Remember then, Philothea, to make occasional retreats into the solitude of your heart, whilst outwardly engaged in business or conversation. This mental solitude cannot be prevented by the multitude of those who are about you, for they are not about your heart but about your body : so your heart may remain alone, in the presence of God alone. [...] When the father and mother of St. Catherine of Sienna had deprived her of every opportunity of place and leisure to pray and meditate, our Lord inspired her to make a little interior oratory within her soul, into which, retiring mentally, she might amidst her exterior affairs, have leisure in this holy solitude of the heart ; and when ever the world afterwards assaulted her, she received no inconvenience from it, because, as she said, she shut herself up in her interior closet, where she comforted herself with her heavenly spouse. And so she afterwards counselled her spiritual children to make a chamber in their heart, and to dwell there.
Withdraw, then, your spirit from time to time into your heart, where, separated from all men, you may familiarly treat on the business of your soul with God.
[Text from "Introduction to the devout life", written by St. Francis de Sales]