Roy Bhaskar argued that there are three levels of relative reality: the Real (causal mechanisms), the Actual (the events generated by those mechanisms), and the Empirical (direct sensory observations). The Real is metaphysical. It cannot be directly be observed.
In physics, no one has ever seen gravity. Yet, we know that any object in a vacuum falls at a constant rate of approximately 32 feet (or 10 meters) per minute. No one has ever seen the strings posited by any of the three major string theories. Yet, string theory is one of the dominant perspectives in theoretical physics.
Similarly, human nature cannot be directly observed. It belongs to the Real, not the Actual, domain. To Bhaskar, we discover it, metaphysically, through phenomenological analysis. In other words, because we can observe regularities (such as the ability to learn language), we can speculate that there is a human nature.
Critical realism is, that respect, similar to speculative realism. It rejects the idea, found in direct realism, that reality is limited to directly observable phenomena.