In this paper Popper formulates and discusses a new aspect of the theory of mind. This theory is partly based on his earlier developed interactionistic theory. It takes as its point of departure the observation that mind and physical forces have several properties in common, at least the following six: both are (i) located, (ii) unextended, (iii) incorporeal, (iv) capable of acting on bodies, (v) dependent upon body, (vi) capable of being influenced by bodies. Other properties such as intensity and extension in time may be added. It is argued that a fuller understanding of the nature of forces is essential for the analysis of the mind-brain problem. The relative autonomy and indeterministic nature of mind is stressed. Indeterminism is treated in relation to a theorem of Hadamard. The computer theory of mind and the Turing test are criticized. Finally the evolution of mind is discussed.