or maybe I should use archaic language to make it sound better?
Take your pick, either one works for me. And I suppose I could leave it at that, keep it all oracular and aphoristic and, well, mcluhanesque, but then again maybe it's worth saying a little bit more about what this means.
Let's start with the second part. The admonition to know yourself was said to have been written outside of the Temple of Apollo at Delphi, back in ancient Greece. No one's quite sure who first said, perhaps old Socrates himself, or one of the pre-Socratic philosophers and Ionian physicists such as Thales or Heraclitus, or maybe it was that old mathematician Pythagoras. Although the Oracle at Delphi is mentioned in Homer's epic poetry, and no doubt originates in the oral culture of ancient Greece, the addition of these words, and perhaps the Temple itself, is a product of a literate mindset. Indeed, it is not possible to know oneself without a mirror of some sorts. A mirror image allows us to reflect upon and become self-consicous of our looks. A mirror image of one's mind allows the same to happen regarding our thoughts, and this is exactly what writing provides. The written word lets us spill our thoughts out on a physical writing surface, freeze them to view and review, and it is only in this way that you can begin to know your own mind. Without writing, there is little capacity for introspection.
Writing then gives rise to the notion that we might have an individual self, a self that is distinct and separate from any other, not bound up inextricably in one's family and tribe, but a single self like a single cell. You might say that writing gives us a self to know, but having done so, we do not automatically set about knowing that self. To do so requires extra effort, additional consciousness raising.
Most media work in this way, extending and externalizing part of ourselves, as McLuhan makes clear in Understanding Media. Each medium, then, lets us learn a little bit more about ourselves, lets us see a different angle of ourselves. But even more basically, each medium lets us create a different self altogether, and every new medium leads to the creation of a new kind of self.
At this point I should probably invoke the perspective known as symbolic interaction, pioneered by George Herbert Mead, popularized by Erving Goffman. From this point of view, we do not have one true self, but rather many different selves, each one true in its own way. You are a different self, in large part because you play a different role, in different situations, for example, when interacting with parents, or when interacting with friends, or when interacting with lovers, or when interacting with coworkers, or when interacting with teachers, or when interacting with children, etc.
Each and every role we play is a self we create for ourselves, and we are the sum of the roles we play, the sum of the selves we construct. As H. D. Duncan has put it, we have a parliament of selves.
Each situation also involves a different relationship, and to bring in now the relational theory of Paul Watzlawick, our selves are defined in our relationships. They never exist in isolation. I can only play the role of teacher if there are students who will accept me in that role, and play their complementary parts in the relationship. They play an integral part in defining me as my teacher-self.
So, now, each medium is also a situation, as Josh Meyrowitz has argued in No Sense of Place, and each medium is also a relationship, as Kenneth Gergen shows in The Saturated Self. So, for each medium that you interact through, you construct a different and new role and self (albeit one that may be similar to others in your repetoire). When you are working with social media, this is very obvious at the moment you create a profile for yourself, but the process doesn't end after the profile is finished. In fact, it has only just begun. You create your self though your subsequent communication behaviors, as your relate to and interact with others. This connects, then, to my previously posted point that You Are What You Tweet.
So, in order to know which self you are at any given time, you have to know which medium or media you are communicating within. And to fully know that self, you have to know that medium well, to understand its nature, its biases, its impact and its effects--its media ecology, so to speak.
So, if you know your medium, you can also know yourself, or at least know one part of yourself, and that may be more than many people know... and if nothing else, certainly, it's a start!