“To perceive an object is for that object (in a certain way) to produce in one a sensory experience of it.”
Essentially we see things in a qualitative way. Wee see a straight stick and we see a bent with the illusion of partially submerging the stick in water. Normally things appear correct, but they can appear wrong sometimes. What we get is an adverb like experience: The rim of my coffee cup appears elliptically. It’s round, but if you look at the right angle you would believe it to be elliptical. The idea is that it appears elliptically in this experience, but maybe the next experience it’ll appear round.
The theory differs when dealing with hallucination. Let’s say I see a floating hotdog. Since there is no object (I’m hallucinating), something in that area, another object is producing this visual experience. If this same room was dark, an adverbial theorist would say there was no perception - despite the fake that I would still see the floating hotdog.
The perceptual experience is an intermediator between what our senses pick up and how we perceive in our mind. We don’t see the physical object, but merely the sense datum. An easy way of looking at this is that we see an mental object of our perception. We’re not actually seeing physically outside of our mind, just an image in our mind.
An example of this is a tomato. We have a visual experience which produces an image in our mind. From that image we see that the tomato is red and round. That’s sense data.
This is about the best summation I’ve seen. I perceive a tomato, which looks to be round and red. These are the properties of the perception I see. The existence of this red and round thing is dependent on my mind.
Sense Datum Theory is indirect realism.
The idea here is similar to sense datum theory. We perceive an object and we get data. The book is rectangular, one inch thick, collar blue, many pages of paper, etc. If we take all these pieces of data away - one by one - there will be nothing left. Essentially an object is a collection of sense data.
Phenomenalism is direct realism.
This means that perceptual objects can be seen as sense data, but deny that there is anything beyond our own perceptions of it. There is no world beyond our possible perceptual experiences.