This chapter offers an exposition of Collingwood’s theory of imagination as presented in the commonly overlooked Book Two of The Principles of Art. I show how the standard objections to Collingwood’s view are relatively superficial, and also how the account in Book Two should be understood in the light of Collingwood’s remarks concerning the imagination in his earlier writings (especially Speculum Mentis and Outlines of a Philosophy of Art). For Collingwood, sense perception inseparably involves the imagination of possible objects of perception in any perceptual experience. Moreover, the imagination makes the sensory object thinkable – a position that blends Kantian and Humean motifs. Additionally, the crucial mark of the imaginary object is self-containment (“monadism”), a notion serving to clarify both Collingwood’s claim that the imagination is indifferent to reality or unreality and the conceptual connection, on his view, between imagination and art.