Next... since we agree that we're sharing a common reality, we're clearly experiencing this reality - collecting information about it and then using that information to develop an understanding of it.
Our brain collects information from our senses - taste, smell, touch, hearing and sight - and uses this information to form a mental model of reality.
We often say that our senses can be fooled, however, they only provide information - it is, in fact, the brain that can be fooled. Consider the following optical illusion:
The two tabletops are exactly the same size and shape (feel free to take a piece of paper and trace out the top of one table and then rotate it to match the other). This illusion doesn't fool your senses. Your eyes are sending a very accurate image to your brain - it is the brain that is fooled, because it is making spacial assumptions (for very good reasons).
The same is true for hallucinations. These are generated in the brain. Your brain is convinced that you're seeing, hearing, tasting, smelling, touchin something, but the phenomena exists only in the brain.
Our senses are, actually, very reliable. The model that our brain makes, based on that sensory data, is also very reliable. The fact that the brain can be fooled (and our fascination with making it happen) is, in fact, testament to how reliably that lump of grey matter performs.
We can quantify the reliability of our senses and assess the reliability of our brains - and we do this all the time.
On the right side of my desk I see a plastic bottle of Coke, it appears to be about half-empty, and I can see a very fine layer of condensate around the bottom half. I can test the validity of the information that my eyes have sent my brain by asking someone else if they also see it. The more "someones" who confirm this, the more likely it is that my eyes have sent good information.
A similar example of this is the standard eye chart. We establish criteria that define 'normal' vision - and this allows us to evaluate an individual's vision. This process is also directly confirmed by the individual as they try out various corrective lenses. They can see the difference. There are other tests for other senses.
However, what if there's no one around to verify what I see? Well, I can move around a bit, and view the bottle from several vantage points, and that will provide more data which will either support or refute what I saw earlier. (It should be immediately obvious if this is simply a picture of a coke bottle, as its dimensions will appear to change as my vantage point changes).
I can also use my other senses. I can touch the bottle - confirming the temperature where the condensate is, confirming the shape, weight and other tactile information. I can take the lid off and hear the 'hiss' of carbonation. I can inhale and see if the smell is similar or different to previous experiences with Coke... and I can taste the stuff.
The point is that our senses provide information and the validity of this information can be evaluated in comparison to our other senses, to the sense of other people and to measuring devices that we've constructed.
This is the process by which we understand reality. It is the reason that we can identify abnormal and defective brains and sensory organs.
In addition to the 5 classical senses, there are additional senses - though any claim about a new sense needs to be clearly defined, and supported by evidence. New senses must also be consistent with data received from other senses as well as independently confirmable in others.
For example we have a "sense of balance". This can be tested, is consistent with other senses and independently verifiable.
Some people have claimed to have ESP (extra-sensory perception) or similar senses and trans-senses. These claims have not proven reliable and cannot qualify as a method of providing the brain with information. They aren't consistent with other senses and, when scientifically tested the overwhelming conclusion is that there's nothing to test - the results, in properly controlled studies - are no better than chance.
Are we still in agreement?