Read the excerpt from Ralph Waldo Emerson's essay entitled "Experience."

The least activity of the intellectual powers redeems us in a degree from the conditions of time. In sickness, in languor, give us a strain of poetry, or a profound sentence, and we are refreshed; or produce a volume of Plato, or Shakspeare, or remind us of their names, and instantly we come into a feeling of longevity. See how the deep, divine thought reduces centuries, and millenniums, and makes itself present through all ages.

According to Emerson, which of these is immune to time?

intellectual activity
sickness
physical activity
memory