"Know Thyself" was such an important concept to the ancient Greeks that it was inscribed at the temple of Apollo at Delphi. Though the aphorism is usually attributed to Socrates, it has been promulgated by philosophers around the world.
Certainly, you don't need to be a philosopher to believe that "knowing thyself" is important. Indeed, the desire to do just that is what draws most college freshman into taking Psych 101. Too bad many of them are disappointed as they learn little about themselves but much about research studies on learning, perception, cognition and attitudes which they don't view as relevant to their lives.
In today's world, people focus "knowing thyself" on their bodies. To know yourself is to know your weight, your body fat, your blood sugar level, your carb intake, your heart rate, your blood pressure, your cholesterol (that's the LDL and HDL), your triglycerides, your C-reactive protein. And no doubt, each year medical science will give us more numbers to know so that we can be more fit and live longer lives.
Though numbers have inched their way into many of the crevices of our lives, it was not numbers that Socrates was speaking about. It was knowledge obtained via introspection and reflection. In the modern world, however, we are so busy with so many things that we don't put much value on reflection. Indeed, we often denigrate it by calling it "navel gazing," "daydreaming" or simply "wasting time".
Furthermore, we don't ingest psychological research the way we do medical research. How many people do you know who have heard of the latest studies on obedience, unspoken prejudice or the proliferation of false norms? And even if you did hear of those studies, chances are you would relate the research findings to the other guy, not yourself. A few examples:
Hence, if your goal is to increase your self-knowledge, you must eliminate the easy answers which are oh, so comforting. Explore beneath the surface. Familiarize yourself with psychological studies. Then, ask yourself, "How does this apply to me?"
Since what we know about ourselves is often just the tip of the iceberg, question your beliefs, attitudes, motives and memory. Explore your emotions (don't just rely on your reasoning) to obtain an in-depth understanding of why you act the way you do.
"Knowing others is intelligence; knowing yourself is true wisdom."~Tao Te Ching