 The present text intends to give a brief (but broad) overview of the interaction between many different philosophical concepts, and to show how the use of multi-modal logical systems can shed some light on these concepts’ interaction. We start in  section 2  (Defining concepts in terms of others) by discussing basic scenarios that, starting from existing systems, use a combination of ‘syntactic’ and ‘semantic’ strategies for defining further concepts. These cases are based on the idea that some notions can be defined in terms of others, with the famous understanding of knowledge as justified true belief being one of the most notable examples. An alternative to this idea is to consider that the involved concepts emerge independently, but are still somehow related, as the case of the relationship between knowledge and time. From a formal perspective, this amounts to looking at the different modes in which two (or more) existing systems can be combined.  Section 3  on General strategies for combining modal systems presents an overview of some of the most relevant strategies. After this slightly technical excursion, the discussion takes a philosophical perspective, describing first combinations of multiple modalities  (section 4  on Significant interactions between modalities), and finishing with examples of cases where the interaction between modalities sheds light on philosophical issues  (section 5  on Multi-modal systems in philosophical discussions).