In terms of "human nature" what is "strength" in your opinion?

What makes us strong, what makes an individual strong? Is there something that makes us stronger together? Are we ever truly stronger together?

These are vague questions, open to your interpretation and opinions.
Update: There may of course be different kinds of strength, and you may not be able to measure "strength" absolutely. There's usually a gray area with most things of which they exist.
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