By “solitude”, Foster means also to include “silence” and as usual warns against excess and vanity:
Finding peace in the desert places is key to hearing the voice of God, but this does not mean to literally move to the desert. For most of us, that is not our calling.
Foster suggests that we try to find “little solitudes” to fill our day. Quiet places at work, outside, and at home can be havens of calm in our otherwise noisy lives.