According to Reid, perception is an operation of the mind by which one becomes aware of external objects. He names three main features that collectively distinguish perception from other mental operations, like imagination, memory and judgement: (1) a conception of the object perceived, (2) an irresistible conviction and belief of its present existence, and (3) a cognitive immediacy regarding the conviction and belief. That is, a perception consists of two components, a conception and a belief, arising independent of reasoning, argument or other mental deliberation. The first feature makes perception intentional or object-taking. The second separates it from operations pertaining to non-existent or past objects, like those involved in imagination and memory. The third makes perception independent of inference. For Reid, perception means drawing on some characterisation of the perceptual object and gaining confidence that the object exists without constructing syllogisms or analysing data. In addition to these three, Reid emphasises the causal chain connecting a perceived object to someone's perception of it. Perception grants you the raw ability to know things and properties outside yourself.

On Reid's scheme, perception deploys conceptions with regard to objects, and in virtue of these conceptions, perception is intentional, having the ability to take objects. A perceptual conception is of the perceived object, therefore the perception is of the object. The intentionality of the perception, its object-taking, arises in virtue of the intentionality of the conception. You might express a conception in verbal-descriptive terms – for example, ‘the computer on the desk’, ‘the dog on the neighbour's porch’ or even ‘that thing left of the tree’. Thus, conception's contribution to the overall perception includes two aspects, an object-taking and a mental categorisation or labelling of that object. It involves thinking of an object as something.

It may seem odd that Reid considers belief in the perceived object essential for perception. Conception alone, like imagination or simple apprehension, is not enough. Conception, on Reid's view, carries no judgement, no conviction, about the existence of any object. So Reid adds belief, a mental act of affirmation or denial, to distinguish perception from mere imaginings or musings.