It is hard for some of us to believe this, but the Neo-Ricardians (NCs) are still with us. In fact there seems to be something of a resurgence. They are still claiming that the phenomenon of reswitching renders the very concept of 'capital' as a factor of produciton meaningless and, that this, therefore, shatters the *entire* foundation of neoclassical economics. How frustrating it is! Not that these ridiculous ideas will not die, but that a whole new generation has now to be taught how wrong and dangerous they are.
The NCs are wrong in a number of ways. 1. they erect a metaphysics of a world in equilibrium and engage in comparative statics - never addressing the question of how actual economies actually work and move from one 'equilibrium' to another. 2. they confuse the price of capital services with the price of borrowing (any kind of service) and call it the rate of interest instead of the rental rate on capital. 3. they never address the fact that billions of people use capital accounting every day and seem to think that capital is a real thing. I could go on.
But perhaps the 'largest' of their errors is to claim that, while the factor of production capital is non-existent, labor is real - in fact what we think is capital is actually (HT Karl Marx) indirect labor. In fact, it is the other way round. It's all capital. All productive services - from human bodies or from machines or buildings or land - are valuable only because they are 'knowledgeable' - they 'know how' to do things that we value - they are 'embodied knowledge' (HT Howard Baetjer Jr.). The simplest and most plausible way to think of factors of production is to think of them as different types of capital - human and physical.