Music is the pleasure the human mind experiences from counting 

without being aware that it is counting.

I maintain also that substances, 

whether material or immaterial, 

cannot be conceived in their bare essence without any activity, 

activity being of the essence of substance in general.

When a truth is necessary, 

the reason for it can be found by analysis, 

that is, 

by resolving it into simpler ideas and truths 

until the primary ones are reached.

I hold that the mark of a genuine idea 

is that its possibility can be proved, 

either a priori 

by conceiving its cause or reason, 

or a posteriori 

when experience teaches us 

that it is in fact in nature.

I do not conceive of any reality at all 

as without genuine unity.

There are also two kinds of truths: 

truth of reasoning and truths of fact. 

Truths of reasoning are necessary 

and their opposite is impossible; 

those of fact are contingent 

and their opposite is possible.