Someone was saying that even if using machines becomes cheaper than using humans, capitalist will still use humans because
"automation constitutes constant capital and human labour is variable capital
The Tendency for the Rate of Profit to Fall disproves that fact"
What do those mean?
One thing to note is that machinery does not impart value onto the things they produce because they can produce so many things for a relatively low fixed cost. Marx equates the infinitesimal value imparted by machinery to that of the forces of nature. This is an important part of why the exchange value of goods produced by machinery drops dramatically.