A dualism is an ontology that proposes things are made of two different kinds of things. No one really does substance dualism anymore (we think that everything is made of the same kind of stuff), but property dualism (the claim there are two different kinds of properties) is alive and well in science. In psychology, this gives us material properties and mental properties, and it's rooted in a broader commitment to a dualism between organism and environment. Turvey explains this with reference to the notion of adaptation in evolutionary theory - the idea that the environment and its niches are what they are prior to organisms, and the job of the organism is to adapt, bring itself into alignment with the environment.
This gets us into causality. In the dualism, the environment causes the evolutionary change in the organism, but the organism does not cause the environment to take certain forms. This comes from Newtonian mechanics, in which the system under study and the environment in which that system operates are treated differently, with different kinds of properties. (Note: rejecting this will be part of rejecting mechanistic approaches to science, which may yet be an error.)
In Newtonian mechanics, systems have states at time t (a position, velocity, etc, plus a mass), and over time systems change states according to recursive functions which characterise the environment of the system imposing forces on the system. The system is the material cause of the behaviour; the environment is the efficient cause, and these are different kinds of causes: a dualism.
With respect to perception, this dualism creates some tensions. For example, the science of environments and the science of perceptual systems tend to live in different disciplines. Second, perceptual science takes the results of the environmental science as it's starting point (i.e. the world as described by physics is what perceptual systems have to work with in order to create perceptual experience). Any gap between what physics offers and what perception accomplishes must be filled by epistemic mediators that do the necessary work. These are internal to the perceptual system, by definition within this framing. Perception is immediately and necessarily indirect (more on this in the next Lecture).
At this point, Turvey jumps straight into describing the very different mechanics he's going to appeal to. Remember in Lecture 1 he claims that complexity theory is going to be the right kind of framework for studying perceptual systems. He discusses a concept from Iberall, CES Spectroscopy, which is just a Turvey way of talking about examining the characteristic behaviours of the system to identify the kinds of subsystems it is made of. This gives us a way to talk about organisms having behavioural modes, preferences among those modes, dynamical stability when exhibiting the modes, and how interacting with the environment perturbs that stability and thus creates the next behaviours.