Logic seems to be concerned with something humans do called thinking. But what does it mean to think? In the sense we want to investigate, thinking does not mean simply any mental activity, such as daydreaming, introspection, memory, anticipation and so forth. Instead something quite specific is meant that we will refer to with the ancient Greek word logos (the root word for logic). To begin to develop a sense for the meaning I want to establish for this word, consider these quotes from Heraclitus (an ancient Greek philosopher):
Of the logos which is as I describe it men always prove to be uncomprehending, both before they have heard it and when once they have heard it. For although all things happen according to this logos men are like people of no experience, even when they experience such words and deeds as I explain, when I distinguish each things according to its constitution and declare how it is; but the rest of the men fail to notice what they do after they wake up just as they forget what they do when asleep.
To those who are awake, there is one ordered universe common to all, whereas in sleep, each turns away from the world to one of his own.
Therefore it is necessary to follow the common; but although the lo/goj is common, the many live as though they had a private understanding.
Listening not to me, but to the logos, it is wise to agree . . . 
Everyone, he says, is like a dreamer in their own private world, not seeing the real world around them. Though his words describe how things are, people instead are lost in their idiosyncratic dreams, limited to their own private perspective. In contrast to these dreamers are those who are awake to the one ordered universe common to all, who have what he calls a public perspective. And this ability to key into the common patterns of the public world—summed up in the word logos—is what he says we should pay attention to if we seek to draw true conclusions about the world around us.
The word idiosyncratic is a combination of two Greek words, idios and synkrasis. It literally meant a private temperament or character. And one limited to the sphere of a private perspective would then be called an idiotēs. An idiotēs is not simply someone who is stupid, but one who is limited to their private perspective on things—one who lacks knowledge of the common patterns of the public world. Take a dermatologist, for example. As the title literally indicated, such a person is one who follows the logos of the dermata, the skin. Such a person has knowledge not just of their private experiences of having skin, their own particular skin—my skin—but instead is able to recognize the patterns of skin in general—anyone’s skin. Such a person the Greeks would have called a sophos—one who knows, who has knowledge. Knowledge is of the universal, and is not limited to the particular. The opposite of being an idiotēs is to be one who pays attention to the pattern or structure common to different realms of experience: someone who, as Heraclitus puts it, listens to the logos.
In ancient Greek logos could mean many things:
word, words; language
a saying or statement
a speech, a conversation
thought or reason
an account or reckoning
And all these things were thought to be the things that made us human—that defined what it meant to be a human being. Perhaps you’ve heard the old definition attributed to Aristotle, that the human being is a rational animal. But this is not what Aristotle said. In The Politics, what he actually says is that:
Among the animals only the human being has the logos.
He says that we have a capacity, a characteristic, an ability, indicated by the word logos. Here we can find a clue by pushing back the history of the word a little further. Logos is a word derived from the word legein), which originally meant:
to pick out
to gather, pick up
to count, tell, reckon up
to recount, tell over again
It’s original meaning simply refers to the ability to pick things out and to pick them up. It was the word used when you went out to gather in the crops—you would pick out what was ripe and pick them up to gather them in as the harvest. What you can distinguish or pick out you can handle and deal with—and to pick things out and pick them up you have to see what is same and what is different in what you are observing. Thinking in the sense we are discussing here begins with being able to distinguish one thing from another and to compare one thing with another—to see that this is different from that, and that this is just like that over there.
So thinking begins with an awareness of sameness and difference—with being able to distinguish one thing from another—to tell them apart. And not just to distinguish them or tell them apart, but to be able to compare them through laying them side by side, so that you can reckon up or count up what you have gathered. With this ability, a new kind of awareness dawns—a self-awareness that engages the world and tells us something new about it in a very distinctive way. From the many, we begin to distinguish the ones, the unities, by listening to the logos.
And you can then give an account—logos—of what you have gathered, and you can hold it in memory and give that account over and over again—you can recount it to others. And so from many things we can begin to see a common pattern that enables us to see them in a new light. We might see that the fall of the apple is like the fall of the moon around the earth; and that the fall of the moon around the earth is like the fall of the earth around the sun.
moon/earth = earth/sun
Or we can do it with numbers:
4/8 = 2/4
We call this giving the ratio, but the Greeks called it giving the logos (ratio is the Latin translation of the Greek word logos).
Or we might notice patterns within patterns as we speak our logoi (the plural of logos):
If I am human, then I am mortal.
If I am mortal, then I will die.
So, If I am human, then I will die.
And if H stands for I am human, M for I am mortal, and D for I will die, then we can play with the patterns:
1. H ⊃M 1. p ⊃ q
2. M ⊃ D /H ⊃ D 2.q ⊃ r /p ⊃ r
There is a gathering of thought which can be held together in memory, and something like Newton’s theory of gravitation can become a permanent possession of the human race, which we can tell from one generation to another—giving the account, and then recounting it: giving the logos. Or on a more modest but no less important scale, the same is true of the pattern of argument about being human and dying given above, which has come to be called the hypothetical syllogism.
This ability to recognize sameness and difference is at the heart of what is meant by thinking in the sense of engaging the logos. It is at the heart of what we call the practice of logic. When you have self-awareness like this you key into a world that is common to all who can think. You join those who can move beyond private things, the self-awareness of particularity, to the great common world Heraclitus characterized with the word logos: the world of the mind, the world of reason, which is open to anyone who takes the trouble to cultivate this awareness. And in this world private opinions don’t matter. You may say you think that such and such is true—but in the common world of reason other men and women of reason will be free to question you, and you will then need to give reasons for what you think: Why do you say this? Is this an argument? What are your premises? What is your conclusion? What is the form of your argument? Does it have a valid form? Are your statements consistent? Are they contradictory? Are you THINKING?
Which is another way to ask: Are you in the common world of knowledge? Are you listening to the logos, or might you be an idiotes, dreaming something to be real when it is not, living as though you had a private understanding? Are you really listening to the logos we hold in common? Are you a man or woman or reason?
And when you operate in this realm, personalities become on one level insignificant:
Listening not to me, Heraclitus says, but to the logos, it is wise to agree . . .
If I can give you reasons that stand up in the broad common world of knowledge, you would be wise to agree with me—even if you don’t like me, my dog, or my politics.
But you would be a fool to listen to me if all I had were opinions offered on my own authority—even if you loved me, my dog, and my politics. You would be a fool to listen to me if I were an idiotes and a dreamer. In the act of thinking, we wake up from the dreams of particularity to the waking world of the one and the many, the same and the different, and we realize an essential part of our humanity in realizing the logos.
 G.S. Kirk and J.E. Raven, The Presocratic Philosophers (Cambridge: 1975), 187.
 Ibid., 188
 See Robert E. Meagher, “Introduction,” Albert Camus: The Essential Writings (New York: 1979), 2-3.
 H. G. Liddell and Robert Scott, A Greek-English Lexicon (Oxford: 1977).
 Meagher, 2-3.
 Liddell and Scott.
 Aristotle, Politics 1.1253a: lo&gon de\ mo&non a!nqrwpoj e1xei tw~n zw~|wn
 Liddell and Scott.
 See Otto Pöggeler, Martin Heidegger’s Path of Thinking (Atlantic Highlands: 1987), 160-161.