(The below notes are pretty much my attempt to summarise the content in this sample chapter from this book. I am posting this in discussion because I don’t think I will get the time/be bothered enough to improve upon this, so I am posting it now and hope someone finds it interesting or useful. If you do find it interesting check out the full chapter, which goes into more detail)

We don’t experience the world directly, but instead we experience it through a series of “filters” that we call our senses. We know that this is true because of cases of sensory loss. An example is Jonathan I., a 65-year-old New Yorker painter who following an automobile accident suffered from cerebral achromatopsia as well as the loss of the ability to remember and to imagine colours. He would look at a tomato and instead of seeing colours like red or green would instead see only black and shades of grey. The problem was not that Johnathan's eyes no longer worked it was that his brain was unable to process the neural messages for colour.

To understand why Johnathan cannot see colour, we first have to realise that incoming light travels only as far  as the back of the eyes. There the information it contains is converted into neural messages in a process called transduction. We call these neural messages: "sensations". These sensations only involve neural representations of stimuli, not the actual stimuli themselves. Sensations such as “red” or “sweet” or “cold” can be said to have been made by the brain. They also only occur when the neural signal reaches the cerebral cortex. They do not occur when you first interact with the stimuli. To us, the process seems so immediate and direct that we are often fooled into thinking that the sensation of "red" is a characteristic of tomato or that the sensation of “cold” is a characteristic of ice cream. But they are not. What we sense is an electrochemical rendition of the world created by our brain and sensory receptors.

There is another separation between reality as it is and how we sense it to be as well. Organisms can only sense some types of stimulus between certain ranges. This is called the absolute threshold for different types of stimulation and it is the minimum amount of physical energy needed to produce a sensory experience. It should be noted that a faint stimulus does not abruptly become detectable as its intensity increases. There is instead a fuzzy boundary between detection and non-detection, which means that a person’s absolute threshold is in fact not absolute at all. Instead, it varies continually with our mental alertness and physical condition.

To understand the reasons why the thresholds vary, we can turn to the signal detection theory. According to the signal detection theory, sensation depends on the characteristics of the stimulus, the background stimulation and the detector (the brain). Signal detection theory says that the background stimulation makes it less likely, for example, for you to hear someone calling your name on a busy downtown street than in a quiet park. The signal detection theory also tells us that your ability to hear them would depend on the condition of your brain, i.e. detector, and, perhaps, whether it has been aroused by a strong cup of coffee or dulled by drugs or lack of sleep.

The thresholds also change as similar stimuli are continued. This is called sensory adaption and it refers to the diminishing responsiveness of sensory systems to prolonged stimulation. An example of this would be when you adapt to the feeling of swimming in cold water. Unchanging stimulation generally shifts to the back of people's awareness, whereas, intense or changing stimulation will immediately draw your attention.

So far, we have talked about how the sensory organs filter incoming stimuli and how they can only pick up certain types of stimuli. But, there is also something more. We don’t just sense the world; we perceive it as well. The brain in a process called perception combines sensations with memories, motives and emotions to create a representation of the world that fits our current concerns and interests. In essence, we impose our own meanings on sensory experience. People obviously have different memories, motives and current emotional states and this means that we attach different meanings to our sensations i.e. we have perceptual differences. Two people can look at the same political party or religion and come to starkly different conclusions about them. 

The below picture provides a summary of the whole process discussed so far (stimulation to perception):

From simulation to perception, there are a great number of chances for errors to creep in and for you to either misperceive or even not perceive some types stimuli at all. These errors are often exacerbated by mistakes made by the brain. The brain, while brilliant and complex, is not perfect. Some of the mistakes it can make include perceptual phenomena such as: illusions, constancies, change blindness, and inattentional blindness. Illusions, for example, are when your mind deceives you by interpreting a stimulus pattern incorrectly. There are also instances of ambiguity in which some people see a particular colour and others another. This occurs even with people who are not colour blind. It occurs because the brain strives for colour constancy which is seeing the same object as having the same colour under varying illumination conditions. But, this process of colour constancy is not perfect. It is troubling that despite all we know about sensation and perception many people still uncritically accept the evidence of their senses and perceptions at face value.

Another important aspect of perception is that the different types of sensory stimuli, e.g. hearing and vision, need to be integrated. This process of sensory integration can be another source of perceptual phenomenon. An example of this is the McGurk effect in which  the auditory component of one sound is paired with the visual component of another sound. This leads to an illusion, i.e. the perception of a third sound which is not actually spoken. You have to really see (or hear) this in action to understand it, so take a look at this short video which demonstrates the effect.

That was a quick summary on perception. But, an important question still needs to be asked. Is sensory perception and how its input gets organised in our minds the sole basis of our internal representations of the world or is there something else that might placate any creeping errors from perception? This question was asked by many philosophers. Kant in particular, had a distinction between a priori concepts (things that we know before any experience) and a posteriori concepts (things that we know only from experience). He pointed out that there are some things that we can’t know from experience and instead need to be born with them. The work of Konrad Lorenz, though, pointed out that Kant’s a priori were really evolutionary a posteriori concepts. That is we didn’t learn them, but our ancestors did. We might believe X despite not having seen it with our own eyes, but this is only because our ancestors who believed X survived. If we couldn’t navigate the world because our internal representations of the world were too distant from how the world actually is, then we would have been less likely to survive and reproduce. What this means is that we can have a priori concepts i.e. innate knowledge. But, that this innate knowledge is itself based on sensory perceptions of the world, just not yours. The types of a priori knowledge can be differentiated into the naturalistic a priori and the inference-from-premises a priori.