@khandro
re: 09:01 Fri
These days, I think that goes without saying. Maybe, in Eliot's day, no-one appreciated that situation, or had thought that. Someone had to be first to say it. Eliot evidently went one stage better and got it in print.
Everything in the brain is a "construct". Our senses feed us a stream of information and the brain has to construct a narrative to make sense of the world. "Action X has consequence Y" is both a narrative and a "theory", as you put it.
It doesn't have to be correct ("truth"), it just has to satisfy the brain's sense of what is around it, as an aid to rapid decision making, at a later date, such as when circumstances resemble those from the past; responses to danger, for example.
So, it is a rationalisation machine. Theories must be constructed, tested and disposed of when found to be unhelpful. Also, it helps to think about things once and carry that rationalisation for life rather than thinking about things afresh, every day, especially when there is work to do.
Philosophy is the luxury of the idle rich. They didn't exist until we had farming. No amount of thinking will gather in the crops. ;0)
By "private truths", maybe he is touching on the impossibility of truly knowing what is going on in the mind of someone else:-
An object could be a certain colour (an objective truth, amenable to mechanical measurement, if need be), two people might both describe it with the same word, even down to what shade it is but neither they or any external observers can 'read' their internal mental experience of that colour. Given paints (or a computerised pallete), they could find what matches for -them- but the external observer's perception filter will still be applying its own interpretation and isn't necessarily triggering an identical experience for that observer.
There is an ojective truth (spectrophotometer shows peaks at various wavelengths) but it is "incommunicable" between people except by clumsy word tokens or artistic depiction.
Even when experiences are identical, it would be interesting to know whether this is because some neural pathways are connected up the same way for everyone (activity mapping will evidence this) or despite any subtle differences (we were all taught what colour was what, in infancy, whether we remember that or not).
I chose an over-simplistic example there, trying to get across that we can agree on subjective experiences of a colour, without having to have neural connections as similar as clones. Brains arrive at similar sensations in multifold ways. (Speculation alert).
When it comes to more sophisticated matters, it becomes clear that having individuals all building their own theories of how the world works, how society works, is not conducive to good teamwork. Good teamwork is what got one tribe to win out over its neighbours. Getting all members to think alike aids organisation but at the cost of abdicating some personal control and handing it to a leadership.
Sacrificing freedom for security: there's a Ben Johnson quote about that.