Absence of evidence and evidence of absence

Is it true that absence of evidence is not evidence of absence? The best answer is: It depends on the case. Let us consider three cases:

I. There is no evidence for the existence of extraterrestrial life, but the absence of evidence is not evidence that there is no extraterrestrial life. This is because we may have reason to believe that it is improbable for life not to exist somewhere other than Earth.

II. You are investigating a murder case and there are only two persons, A and B, who might have motivation for murdering the victim. A and B are the only suspects; you have some (but not overwhelming) evidence that A committed the crime, but you do not have any evidence that B did it. The absence of evidence that B did it is not evidence that he didn't do it --- he is still a suspect. However, if later on there is more and more evidence that it was A who committed the crime, this will allow you to give more weight to the fact that there is no evidence that B did it, and you can accordingly say that the absence of evidence is now evidence that he didn't do it.

III. Suppose Jennifer believes that there are unicorns. You point out that unicorns are mythical creatures and there is no evidence that they exist. She responds by insisting that the absence of evidence for the existence of unicorns is not evidence that they don't exist. Is this a reasonable response? No, for we have very good reasons for believing that there are no unicorns and there is no need for having any evidence for the truth of the belief.

There can be a more systematic treatment of "Absence of evidence is not evidence of absence"; the point I want to make here is just that we should not accept it as a general principle.

How was Jesus sacrificed when he did not really die?

Christians believe that Jesus died for our sins, but they also believe that Jesus rose from the dead, that is, Jesus did not die after all. If Jesus did not really die, then he wasn’t really sacrificed, was he? Cf. If you slaughtered an animal in order to sacrifice it to your god(s), but it came back to life immediately, was the animal sacrificed?

Indeed, if you believe that Jesus is God, and that God cannot die, then you have to believe that Jesus cannot die either. You can at most believe that that human organism which was Jesus' body ceased to function and then was revived by God. (This was, by the way, no big deal for God. Remember, God is omnipotent!) But then this would be compatible with Jesus the person not being dead at any time. If Jesus is God, then there was no point of time at which the person Jesus --- who is God --- was dead.

So, strictly speaking, Jesus was not sacrificed; it was Jesus' body that was sacrificed (even this, as I pointed out in the first paragraph, is questionable since his body was revived). But there is no reason to believe that that particular human body was so special that the sacrifice of it was sufficient for redeeming us, for God could easily have taken on another human body.

It may be objected that if, as Christians believe, humans have an immaterial soul, then in the sense in which Jesus did not die, humans whose bodies ceased to function did not die either, nor, for that reason,could any of them be sacrificed. True, but that only strengthens my point. If not even humans can be sacrificed, then how could Jesus be sacrificed? (There is, however, an important difference between normal humans and Jesus: even if humans have an immaterial soul, a human soul can be destroyed by God, while Jesus cannot cease to exist.)

Love at first sight

Is there such a thing as love at first sight? I think it depends on how literally we take the word 'love' here. If 'love' here means no more than a feeling of romantic attraction, then there is certainly love at first sight. However, if 'love' here means, well, love, then it is not clear that most of the cases that people would describe as 'love at first sight' are really love.

When someone describes his first encounter with his beloved as 'love at first sight', it is usually a retrospective description. It is in the light of his love for her now that he sees the first encounter that way. If after the first encounter their relationship had not developed further, that would not have changed that experience, but he probably would not have described it as 'love at first sight'. So it is more reasonable to think that the first encounter was not love at first sight; it is just that it is seen as love at first sight retrospectively.

The largest class ever

It is unusual for a philosophy book written by a famous professional philosopher to be a New York Times bestseller, but here is one: Michael Sandel's Justice: What's the Right Thing to Do?. It is based on Sandel's "Justice" course at Harvard, which has been one of the most popular courses at Harvard since it was first offered about twenty years ago. The class is usually very large, and in Fall 2007, it had 1.115 students, making it the largest class ever at Harvard. I can't imagine what it is like to be teaching such a huge class. I am teaching a 120-student class this semester, and it is already too large for me. My teaching has been improving though. This is the fifth week of the semester and I think I have learnt a few tricks to teach the class more effectively. But it is still not easy. Over a thousand students? No way!

By the way, I have skimmed through Sandel's book and got the impression that it is a very interesting introduction to moral philosophy.

Surprise

Whoever knows he is deep, strives for clarity; whoever would like to appear deep to the crowd, strives for obscurity. For the crowd considers anything deep if only it cannot see to the bottom: the crowd is so timid and afraid of going into the water.

So true. But who says this? Surprise, surprise: it's Nietzsche!

Is everyone a philosopher?

Is everyone a philosopher? For me the answer is clearly 'No'. Some may think we can define 'philosopher' so loosely that anyone who thinks about how to live and answers the question --- who thereby has her own 'philosophy of life' --- is a philosopher. But the fact is, not everyone thinks about how to live. Some people are incurably unreflective; they should not be considered philosophers even on the above extremely loose definition of the term.

There are, however, people who think that even if they are not philosophers, they will understand philosophy if they try to. They believe that if they pick up a philosophy book and read it, they will understand it; they also believe that if two philosophers are discussing a philosophical problem, they can easily chime in and contribute to the discussion. They have no idea that some philosophical ideas/problems/arguments/theories are impossible to understand without some training and background knowledge.

How do I know there are such people? From the following experience: I was reading a rather difficult philosophy book; a friend saw that and asked me what I was reading. I told him the title, and he went on to ask me what the book was about. I tried my best to explain the problem discussed in the book in terms that he might understand. His response was, "How interesting! That will be the next book on my reading list." And I said, certainly unwisely, "I think this book is a bit too advanced for you." He seemed offended.

It is hard to imagine that this would have happened if I were a physicist and the book I read were on quantum mechanics.

Living and understanding life

"Life can only be understood backwards, but it must be lived forwards." This famous saying of Kierkegaard may seem obviously true: the future is indeterminate, so 'my life in the future' is not something definite that I can understand now; by contrast, my life in the past is fixed, but it is gone now and I can only live in the present, moving forwards into the future.

However, I can have a plan for my future and understand what my life should be like according to the plan. This is, in a sense, understanding life forwards. On the other hand, I cannot live without being affected by my memories of the past. If my memories are so powerful that they dictate how I live my life now, then I am in a sense living in the past, living my life backwards.

There is indeed an entry in Kierkegaard's journal in which he remarks that "life must be understood backwards", but what he says next is a qualification of the remark:

And if one thinks over the proposition it becomes more and more evident that life can never really be understood in time simply because at no particular moment can I find the necessary resting-place from which to understand it --- backwards.

There is no resting-place because my life changes constantly for as long as I live. How I understand my life so far depends on how I see things and how I live my life now, but both of these can change (and usually not as a result of my own decision), and my understanding of my life will change accordingly. My understanding of my life is thus always tentative.

Ignore, ignorant, and ignorance

Is "ignore" the root of "ignorant" and "ignorance"? I suppose etymologically the three words must be related, maybe having the same Latin root, but in English "ignore" forms an interesting contrast in meaning with "ignorant" and "ignorance". Ignorance of X is lack of knowledge of X, but to ignore X one has to know of X in the first place, that is, one cannot ignore something that one is ignorant of.

However, if one ignores X, one will not be able to know X further than one already does. So, in a way, ignoring X does lead to ignorance of X. On this understanding, knowledge is a matter of degree, and so is ignorance.

Philosophy and masturbation

There are two things Karl Marx says about philosophy that I think every philosopher should give some thought to:

(1) Philosophy is to the real world as masturbation is to sex.
(2) The philosophers have only interpreted the world, in various ways. The point, however, is to change it.

I don't think (1) is true of all philosophy, but it is easy to philosophize in such a way that all we are doing is no more than finding our way out of a conceptual maze we have created, a maze that does not tell us anything beyond itself. It would be fun, intellectually, if we got out of the maze, and in a sense this would be solving the problem. But we should also ask: Why does it matter?

As for (2), I am not sure philosophers should always try to change the world. On the other hand, it is not unreasonable to think that some philosophy (such as social and political philosophy, normative ethics, and even some philosophy of mind) has the potential to change the world, though it may take a long time.

(1) and (2) are, of course, related. If one's philosophy is nothing but conceptual masturbation, it should be not expected to be able to change the world.