All the mind's activity is easy if it is not subjected to reality.

The line contrasts the mind’s effortless freedom in imagination with the resistance imposed by lived circumstances. Proust often explores how memory, desire, and aesthetic reverie can feel boundless precisely because they are insulated from the constraints of time, other people, and material limits. Read this way, the quote suggests that thinking, fantasizing, or theorizing can seem “easy” when it does not have to answer to facts, consequences, or the stubborn complexity of real life. It also carries an implicit warning: mental constructions—plans, ideals, self-images—gain their seductive clarity by avoiding reality’s friction, and they are tested (and often undone) when confronted with experience.