her term for information, out of the broader formulation. and that the physical measure of entropy was also a measure of the Consider propositions A and B. Using Boolean algebra, we may quantity of information or degree of organisation of the correspond- construct new propositions from A and B by conjunction, disjunc- ing physical system. More precisely, the difference in the quantity tion and negation, as in Equations 4.1 to 4.3. of information between two states of a physical system is equal to the negative of the corresponding difference in entropy of the two AB = Both A and B are true (4.1) states. In nature it is found that self contained systems change from more A + B = At least one of the propositions is true (4.2) highly organised structures to less organised ones, i.e. from states of higher information content to states of lower information content. A = A is false (4.3) It is important to note that in the above context a physical system corresponds to many different thermodynamic systems, and entropy The conditional probability that A is true given that B is true, is a is not simply a property of the system, but of the experiments chosen real number between 0 and 1, and is represented by the symbol to undertake on the system. For example, one normally controls a P(A|B). Therefore p + B CD) is the probability that at least set of variables for the physical system, and one measures the one of the propositions (or hypotheses) A,B, is true, given that both entropy for that set. A solid with N atoms has approximately 6N C and D are true, etc. degrees of freedom, of which only a few, e.g. pressure, temperature, In this formulation of probability as a state of knowledge, or a magnetic field, are usually specified to obtain the entropy. By representation of incomplete knowledge, all probabilities are condi- increasing this set, one obtains a sequence of entropy values, each tional on some information, and there does not exist an absolute corresponding to the chosen constraints. probability. According to Jaynes (