A common saying holds that you are the sum of the five people you spend the most time with. More simply, the people you surround yourself with matter. They are the sounding boards for your ideas and the key forces which shape your growth.

Growth is achieved only when you are challenged. It isn't a single-track process but occurs across four dimensions (which are simply the four dimensions of self):

1. Physical
2. Intellectual
3. Emotional
4. Spiritual

Most of the challenges you will receive across these dimensions are laid extrinsically by others because it's basically impossible to change a system without some form of external stimulus. People can challenge you via three modalities:

1. Inspiration: They inspire you by exhibiting a behavior you aspire to enable in yourself.
2. Collaboration: They offer to work with you towards a shared goal.
3. Direct Challenge: They lay a gauntlet at your feet and say "show me".

Inspiration is the wind at your back and it's maximized by putting yourself in an environment where you are constantly inspired by the people around you. Moving to San Francisco and choosing to live in a co-op based around the goal of "mutually assured non-complacency" were two big steps towards my doing this and I'm endlessly curious to find people who inspire me, whether they are intellectuals, artists, entrepreneurs, or pretty much anyone living their life fully engaged.

To optimize for collaborative challenge, join forces with people who are moving in the direction you're hoping to go. I recently completed a triathlon (which was totally outside my realm of expected possibilities) because a housemate proposed it months ago and asked if anyone wanted to join. Some of the most formative experiences I've had so far began with someone asking "Why not?" and opting to trade Dollar Beer Night for a far more interesting set of possibilities.

Some people respond better to direct challenge than others and it's quite rare to find someone who cares enough about you to provide a direct mandate for change. Hold onto those friends who are capable of looking you in the eye and challenging you to be better. In a world of non-confrontation where everyone is handling each others' egos with kid gloves, the most refreshing phrases to hear are "I bet you can't…" or "It seems like you could have…" or just "You should…".

In the end, the meaningful people in your life extend well beyond just the five who "define you" so it pays to be considerate of everyone who inhabits your orbit. If your goal is to grow, surround yourself with people who inspire you to be your best self, who work with you to achieve your goals and who care enough to provide you with a mandate to change.

Count yourself supremely fortunate if you've found those who can do all three.