When a body “encounters” another body, or an idea another idea, it happens that the two relations sometimes combine to form a more powerful whole, and sometimes one decomposes the other, destroying the cohesion of its parts. And this is what is prodigious in the body and the mind alike, these sets of living parts that enter into composition with and decompose one another according to complex laws. The order of causes is therefore an order of composition and decomposition of relations, which infinitely affects all of nature. But as conscious beings, we never apprehend anything but the effects of these compositions and decompositions: we experience joy when a body encounters ours and enters into composition with it, and sadness when, on the contrary, a body or an idea threaten our own coherence.
—Gilles Deleuze, Spinoza: Practical Philosophy (via heteroglossia)