"Humans are different in private than in the presence of others. While the private persona merges into the social persona in varying degrees, the union is never complete. Something is always held back"

This quote by Brian Herbert suggests that humans have 2 distinct personalities: a personal persona and a social persona. The private personality is the one that is only seen by oneself, while the social persona is the one that is seen by others. The 2 personas combine in differing degrees, but never ever entirely. This recommends that human beings are always holding something back, even when in the presence of others. This might be an idea, feeling, or opinion that is not shown others. It is important to bear in mind that people are complex and that there is always something that is not seen by others. It is necessary to be familiar with this and to be mindful of how we present ourselves to others.