The proper study of mankind is man.
Plac'd on this isthmus of a middle state,
A being darkly wise and rudely great:
With too much knowledge for the sceptic side,
With too much weakness for the Stoic's pride,
He hangs between; in doubt to act, or rest;
In doubt to deem himself a God, or beast;
In doubt his mind or body to prefer;
Born but to die, and reas'ning but to err;
Alike in ignorance, his reason such,
Whether he thinks too little or too much:
Chaos of thought and passion, all confus'd;
Still by himself abus'd or disabus'd;
Created half to rise, and half to fall;
Great lord of all things, yet a prey to all;
Sole judge of truth, in endless error hurl'd:
The glory, jest, and riddle of the world!"
-Alexander Pope, "Essay on Man," Epistle II, ll. 1-18
The way Lem's novel seems most often to be interpreted is to suggest that the story simply affirms a sentiment very like Pope's, that self-knowledge is at least the precondition for any further knowledge of nature or God, if it does not mark the absolute limits of our knowledge. Pope jumps from can't to oughtn't and admonishes that human limitations on what we can possibly know should also be limitations on what we should hope to know—the proper, in both the senses of belonging and suitability, study of mankind is man.
Lem is a bit more of a pessimist in terms of what degree he believes that self-knowledge to be achievable. His depiction of men struggling in terror with the possibility that the planet Solaris can know them thoroughly, shocking them into a form of self-knowledge that is also a self-alienation, suggests that perhaps the study of man might be appropriate for mankind, but not something it can lay claim to with any authority.
Yet I think there is a deeper divergence from the "proper study of mankind is man" line: Lem's novel offers a tantalizing vision of an intellectual problem so large and alluringly complex that human intellectual energy can simply be poured endlessly into its depths. This is what was so exciting to me: the imagination of entire centuries, generations of lives, of research and theorization, an edifice of thought and experiment that Lem fabricates so well. Solaristics is a sort of second Enlightenment for humanity, and Lem makes it convincing as a project, a sustained effort that envelops humankind, or at least as much of it as Lem allows us to see.
Lem, particularly in the passage I excerpted before, also is quick to suggest that we might imagine this project as exceptionally wasteful, a total intellectual loss. And that is almost more thrilling, that this edifice of thought and experiment is at the very least transitory, if not entirely a castle of the air, modeled clearly on the symmetriads that Solaris projects onto its surface from time to time. The novel is a terrifying but wondrous evocation of humanity's ability to attach itself to futilely grandiose projects. The possibility of a "proper study" is itself called into question. Humanity does not, contrary to Pope, "hang between; in doubt to act, or rest," but is over-eager to act, and will keep acting until the edifice topples or subsides back into itself. But again, this is what is most exciting about the novel, most exciting about Solaristics: the depiction of humans throwing themselves into the breach again and again, even at times imagining breaches when there are none.
Pope imagines humanity's fate to be, pitiably, "Born but to die, and reas'ning but to err," but Lem sees beyond that and suggests that mortality and fallibility are part of a larger system of multi-generational human effort that compensates for and corrects (to some extent) both mortality and fallibility. Yet there is no certainty, despite the corrections and compensations of multiple generations of effort, that the project will be, in any sense, "proper."