Our first topic in the study of agency is the intuitive notion of information as a semantic range of possibilities, widespread in science, but also a common sense view. For this purpose, we use epistemic logic, proposed originally for analysing the philosophical notion of knowledge (Hintikka 1962). While the latter use remains controversial, we will take the system in a neutral manner as a logic of semantic information. More precisely, the hard information that an agent currently has is a set of possible worlds, and what it ‘knows’ is that which is true in all worlds of that range (van Benthem 2005c). This picture makes knowledge a standard universal modality. We present some basics in this chapter, stressing points of method that will recur. We refer to Blackburn, de Rijke & Venema (2000), Blackburn, van Benthem & Wolter (2006) for all details in what follows. We add a few special themes and open problems, setting a pattern that will return in later chapters.

The topics to come reflect different aspects of a logical system. Its language and semantics provide a way of describing situations, evaluating formulas for truth or falsity, engaging in communication, and the like. This leads to issues of definition and expressive power. Next there is the calculus of valid reasoning, often with completeness theorems tying this to the semantics. In this book, this theme will be less dominant. To me, modal logics are not primarily about inferential life-styles like K, KD45, or S5, but about describing agency. Finally, there is computational complexity. We want to strike a balance between expressive power and complexity of logical tasks (cf. van Benthem & Blackburn 2006). This comes out well in a procedural perspective on meaning and proof in the form of ‘logic games’ – an interesting case of interactive methods entering logic. Our presentation is not a textbook for epistemic logic, but a showcase of a standard logical system that we will ‘dynamify’ in this book – not to replace it with something else, but to make it do even further things than before.