9. Having in the preceding sections given the Definition of Value, and found that its Form or Cause resides exclusively in the Human Mind, the last branch of our inquiry is to determine the General Law of Value: or the General Equation of Economics : that is to discover a single General Law which governs the exchangeable relations of all Quantities, whatever their nature, at all times, and under all circumstances.

The acknowledged principles of Inductive Science shew us that there can be but one General Law of Value. We have seen in the preceding chapter that there are three distinct species of Economic Quantities, and we have generalised the fundamental Conceptions of the science so as to grasp all these Quantities. Now these three species of Quantities can be exchanged in Six different ways. Our present object is to investigate a General Equation which shall be applicable to all the Six species of exchanges indifferently. The Law which governs the exchangeable relations of material products, must equally govern the exchangeable relations of Debts.

Suppose we make £ the general symbol of an Economic Quantity - that is to say anything whatever whose Value may be measured: and representing these various species of Quantities under the general symbol £, we may affirm that there can be but one Cause of Value for them all; which we have shewn to be Demand: and there can be but one General Law which governs their exchangeable relations.

Now let A and B be any two Economic Quantities of any form whatever, then supposing that A remains the same while B varies with respect to A - then B or the value of A, will vary from four causes : -

It will Increase -

1. From a Diminution in Quantity.

2. From an Increase of Demand. It would Diminish -

1. From an Increase of Quantity.

2. From a Diminution of Demand.

Now as the variations of Value of the other Quantity are influenced by exactly the same four causes, it is quite clear that. the variations of both Quantities will be influenced by eight independent causes: and if these be connected in the form of an Algebraical Equation, that will manifestly be the true General Equation of Economics.

The above is the full expression of what is commonly called the Law or Supply and Demand. It means this that no change can take place in the Exchangeable relations of any two Economic Quantities unless there is some change in the Intensity of Demand or the Limitation of Supply, of one, or both of the two Quantities.