Water is the softest thing, yet it can penetrate mountains and earth. This shows clearly the principle of softness overcoming hardness.
— Lao Tzu

I think there is only one quality worse than hardness of heart and that is softness of head.
Theodore Roosevelt hardness quote

The most spiritual men, as the strongest, find their happiness where others would find their destruction: in the labyrinth, in hardness against themseleves and others, in experiments. Their joy is self-conquest. Difficult tasks are a priviledge to them; to play with burdens that crush others, a recreation. They are the most venerable kind of man: that does not preclude their being the most cheerful and the kindliest.
— Friedrich Nietzsche

Every man of action has a strong dose of egoism, pride, hardness, and cunning. But all those things will be regarded as high qualities if he can make them the means to achieve great ends.
— Charles De Gaulle

The work of art, just like any fragment of human life considered in its deepest meaning, seems to me devoid of value if it does not offer the hardness, the rigidity, the regularity, the luster on every interior and exterior facet, of the crystal.
— hardness quotation by Andre Breton