The Need For Opposition

Immanuel Kant, Lectures on Pedagogy 1803, tr. Robert B. Louden, p. 443, in: Anthropology, History, and Education (Cambridge, 2007):

But a tree which stands alone in the field grows crooked and spreads its branches wide. By contrast, a tree which stands in the middle of the forest grows straight towards the sun and air above it, because the trees offer opposition.