“Without contraries is no progression”
Tennessee Williams says much the same sort of thing in “The Catastrophe of Success”, the essay he wrote about getting ‘soft’ after the success of The Glass Menagerie. Conflict in the crucible is a proving ground of sorts and certainly is a built-in part of process and experience. How one reacts to it affects character and choices ultimately. And often, as remarked on before, the individual is in conflict with certain contexts as well (e.g., Hamlet in Claudius’ court).
I believe the contraries Blake speaks of is also related to a dualistic view (“Fair is foul and foul is fair” revisited–opposites, as Shakespeare shows in many plays likewise, reveals or develops character. Both sides/poles are inevitably true in their various ways, and need to be accounted for (with perhaps life and death being the most basic contraries), but what I said before about how one acts in response to information can also be true when people respond to conflict. Both situations reveal character and who we are, and, as I have said before likewise, these situations reveal a person’s attitude. So much depends upon attitude. And the proverbial view of the glass half full or empty. Indeed, the attitude is the individual, reveals the personality and character of the individual. So contraries, therefore, present opportunity and lead to new possibilities. In this sense, Blake is right. The contraries and conflicts lead to progression and often positive change.