Nietzsche makes his remark about the abyss (in Beyond Good and Evil §146) just after cautioning the reader that someone who fights monsters risks becoming a monster himself.
That can happen to the man of ressentiment. He’s convinced that his various disabilities are caused by someone or something out to get him, and that if only the scourge were eliminated from the world all would be well.
If this is your attitude, Nietzsche is saying, you’re going to get really good at ferreting out the nasty parts of life, wherever they might be hiding, and you’ll uncover one hitherto unrecognized injustice after another: first racism, then structural racism, then elitism, then heteronormativism, ableism, lookism, microagression…. You may get to the point where you can see nothing but monsters.
Someone who sees nothing but monsters can’t fail to develop an affinity with monsters. You may become the kind of monster known as a “moral saint.”
Now, should we interpret what Nietzsche says next, about the abyss gazing into you, as a restatement of this thought? An amplification of or addition to it?
Well, the theme of this aphorism, to judge by the first sentence, concerns the hazards of the life of the free spirit – the one who undermines existing values and creates new values of his own.
Nietzsche seems to mean that if your highest value is truth and you pursue it at the expense of everything else, you risk encountering truths that cause you to question the value of life.
This kind of truth can potentially drain you of energy and inhibit your will to power, which Nietzsche thinks is the most valuable thing there is.
The healthiest human being, Nietzsche thinks, is one whose sheer love of life is so powerful that he or she enthusiastically desires the eternal repetition of everything that has happened and will happen – good, bad, and evil. Such a person affirms life without qualification, absolutely.
Only a person like this can afford to know all the truth. But how do you know whether you’re that kind of person? Or whether there are some truths about the human condition that, if you acknowledged them, would inhibit your love of life?
The only way to find out is to stare deeply into the abyss of truth. An abyss is something that is bottomless: there’s no end to the search for truth. The issue is where the bottom is for you. At some point you may find that the abyss is staring back at you, and then you’ll know you’ve hit bottom.
What Nietzsche is saying could be construed as “Know your limits – be cautious.” Or he could be construed as asking “How strong are you? You’ll never know unless you search the abyss.”