There are two distinct types of physical constraint (and hence two distinct senses of emergence). The first type, which I will refer to as non-constitutive, would be acknowledged by any reductionist. These, typically, are the constraints that describe non-essential properties of physical entities. The height from which I drop an object, for example, is a constraint on the law by means of which I may determine the time it takes to hit the ground. Reductionists, while granting this limited sense of non-derivability, would claim straightaway that it is trivial, and would not prove helpful in explaining emergence as that term is understood in the broader debate, where it describes the novel properties of complex, organized physical entities, such as are found in biology or even chemistry.
Constraints of the second type, which I will refer to as constitutive, are likewise not derivable from physical laws alone, but they underlie a second, more substantial sense of emergence, for they constitute physical entities themselves. The structure that establishes a physical entity is defined, I will argue, by specific boundary conditions or constraints on physical laws. Complex structures are in fact sets of such interdependent constraints. This concept of constitutive constraint can function as the basis for a (non-trivially) non-reductive physicalism.

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