"The tendency of the rate of profit to fall (TRPF) is a theory in the crisis theory of political economy, according to which the rate of profit—the ratio of the profit to the amount of invested capital—decreases over time. This hypothesis gained additional prominence from its discussion by Karl Marx in Chapter 13 of Capital, Volume III,[1] but economists as diverse as Adam Smith,[2] John Stuart Mill,[3] David Ricardo[4] and William Stanley Jevons[5] referred explicitly to the TRPF as an empirical phenomenon that demanded further theoretical explanation, although they differed on the reasons why the TRPF should necessarily occur.[6]"
Now, the theory is actually much more simple than whatever nonsense is described above.
The theory simply says:
Capitalist profit is made from earnings minus costs.
Costs are wages and non-wage costs such as machines and materials.
As machines and other non-wage inputs in production increase, only two things can follow:
1. Decrease in wages cost (reducing wages or reducing number of workers)
2. Decrease in profit
Both of these are essentially harmful.
They can both only be avoided if non-wage costs never increase.
If non-wage costs increase, to maintain same profit rate, either some workers must lose jobs or wages must decrease. Otherwise, rate of profit decreases.
So, why do non-wage costs increase?
Some think its due to technological advancement. As technology advances and new machines and robots are put to use, what happens is that more money needs to be payed for those.
Others think that an increase in the cost of raw materials contributes. Since economy is built on mass production, every increase in rate of production by technological advancement leads to greater use of resources which in return increase the price, even more so for resources which are being depleted faster or in cases where their supply isnt sufficient to cover all,
and thus since demand usually exceeds supply in case of resources who with advancement of technology become more in demand, but less in supply,
What follows is general rise in non-wage costs of production which can only result in fall of profit or fall of wages.