In Chapter 1, I start by analysing the role of generalisations in scientific practice. Law statements or generalisations are involved in one way or another in explanation, confirmation, manipulation or prediction. I argue that these practices require a particular reading of the generalisations involved, namely, as making claims about the behaviour of systems. These practices therefore presuppose the existence of systems or things. Next, I look at the modal surface structure associated with laws. I use the term ‘surface structure’ to indicate that this structure may or may not be reduced to non-modal facts – as the Humean has it. I will sideline the debate about whether Humeanism is a tenable philosophical position. The positive claim I advance is that the modal surface structure can be explicated in terms of invariance relations – where I take invariance to be a modal notion.