The prevalence and complexity of human-computer interaction makes a general understanding of human cognition important in design and development. Knowledge of some basic, relatively simple, principles for human brain function can significantly help such understanding in the interdisciplinary field of research and development Human-Computer Interaction (HCI) where no one can be an expert at everything. This paper explains a few such principles, relates them to human-computer interaction, and illustrates their potential. Most of these ideas are not new, but wider appreciation of the potential power of basic principles is only recently emerging as a result of developments within cognitive neuroscience and information theory. The starting point in this paper is the concept of mental simulation. Important and useful properties of mental simulations are explained using basic principles such as the free-energy principle. These concepts and their properties are further related to HCI by drawing on similarities to the theoretical framework of activity theory. Activity theory is particularly helpful to relate simple but abstract principles to real world applications and larger contexts. Established use of activity theory as a theoretical framework for HCI also exemplifies how theory may benefit HCI in general. Briefly, two basic principles that permeate this perspective are: the need for new skills and knowledge to build upon and fit into what is already there (grounding) and the importance of predictions and prediction errors (simulation).