OK, until now we saw that all phenomena in chemistry and biology can be described by physics alone, but in some complex and perhaps longer description manner. To understand these descriptions much easier we need to hide some details and define some new - which I say nonphysical - definitions. These definitions somehow hide physics and create some new science by themselves. And as we saw these abstractions were chemistry and biology by now.
Although these abstractions may reduce the accuracy of scientific research, but they make the research more feasible and understandable. The thing I'm trying to say is that it is crazy to measure the length of the new world trade center height which is about 541 meter with a micrometer, which is a tool to precise measurement of things. You have to use some abstraction to do this, you have defined 1 meter which is 1,000,000 micrometer. So with a metering tool, you can measure the height in a more convenience way.
So what we are going to talk about here is something which may seem a little odd, psychology also obeys physics rules. In fact, if you know exactly and enough of anything you can predict when two people are going to talk what or how they will talk about.
Is human behavior deterministic?
I will talk about "If it is possible to exactly define the future or not?" some day, because what we are going to talk about is somehow related to this, for now, the question is "Are human behavior is deterministic?"
A system is deterministic if the output of the system only depends on the inputs and no randomness involves to making the output or changing the system's state. So the system will produce the same output if you give it same input even millions of times. In fact, I think there is no randomness in the world at all.
Probability theory is exactly an abstraction over some complex calculation we usually don't have enough knowledge and power to do them. For example when you flip a coin, if you exactly know the hit vector information and the functions definition of environmental forces, who says we can't write some equations to find out on which side the coin will come back!?
Agree? Yes you are, if not think about the previous paragraph again. Let us back to our own topic, can we determine the behaviour of a human? Of course, consider a human as a biological creature with extremely complex interaction with the environment they live in, it all can be described by physics. There is also one catch, and it is if you consider a human as a machine who knows the system function of this machine? Who has programed it?
We talked about the brain which is like a billions of simple true/false memory cells. These cells are get their program (being true or false) during human life. The complexity of this program especially when you give same input to it in different conditions is extremely extremely high, so that you even can't have a good prediction - with probability theory - when you give it same inputs frequently. This is why when you say "hello darling" to your friend, sometimes she may kisses you and sometimes she may slap in your face. The reason is although this is your friend in both time, but this machine or system is not at the same state or even the program of the machine is not the same at these two times. So the reason you can't determine the output of "hello darling" is not that human being is not deterministic system, but is that we don't know exact program of the brain and don't know the exact environment parameters values, so we can't determine how she may react by giving this input to this system.
