 Perception was, throughout the Middle Ages, a contentious topic, and it was also a topic in which the answers to strictly causal questions could influence philosophical positions in other areas (for example, on whether certain knowledge of external entities was attainable). The “traditional” view, dating back to Roger Bacon in the mid-thirteenth century, was that physical objects were known because they caused a succession of likenesses, or species, first in the medium between the object and the perceiver, then in the senses, and finally in the intellect, of the perceiver (Tachau 1988, pp. 3ff., Normore 2010, pp. 255f.).  This position was attacked by thinkers such as Henry of Ghent, Peter Olivi,  and Duns Scotus. Interestingly, many of these criticisms tend towards a relational account of perception, in which – although species still play a role – the role that they play is to be a means by which we know things, and in which the species themselves are not known directly but only by reflection. (Tachau 1988, p. 66}. (Panaccio 2010, pp. 242f., Normore 2010, pp. 255f.)