In this book, Stephen Mumford and Rani Lill Anjum present their theory of causal dispositionalism, that is, causation based on dispositional properties, or powers. I highly recommend it to anyone interested in the philosophy of causation.
Powers do the causal work in our world, according to the authors: effects are brought about by powers manifesting themselves, and the manifestation is itself a further power or set of powers.
A central idea is that powers don't necessitate their manifestations - they dispose toward them. Causality has long been associated with the idea of necessity, and necessity (and the sense of constant conjunction) is too strong to describe causation. The main insight here is that other factors can prevent or interfere with the expected manifestation (and, indeed, they often do).
To help demonstrate how a disposition can be enhanced or, importantly, hindered by other powers, the authors develop a vector addition diagram. Only when the sum of vectors (with various strengths and directions) exceeds some threshold do we get the manifestation. They extend the model to more complex scenarios to argue that the model is robust enough to explain non-linear and even "emergent" behavior.
In addition to arguing strongly against necessity, the authors want to overthrow another usual notion. The authors reject as misguided the typical "two-event" conception of causation, where cause is temporally prior to effect, in part because no one has a compelling account of how you get from one to the other. Instead causes and effects are simultaneous - they are two aspects of a temporally extended process which brings about a change.
An important and creative part of the book explores the distinctive modality of dispositions in more depth. Dispositional modality (weaker than necessity but stronger than "pure" contingency) is the primitive and fundamental modality of nature. We derive necessity and possibility from our prior experience with dispositionality. Mumford and Anjum argue that we do indeed perceive causation, and present what they see as the clearest examples of this in the case of bodily sensation and specifically proprioception.
The book concludes with a compelling application: showing how the theory fits with processes studied in biology and genetics.