Our initial attitudes to mum's disease were probably slow to change, but then there was little to respond to. How do you help someone who is losing their memory? You tell them to make lists and get organised! You only later realise that forgetting how to be organised is at the root of the problem, and lists are only possible if you can remember what to put in them. That is just one way in which the progressive degradation of Alzheimer's strips away your assumptions one by one - each time you propose making use of whatever small capability is left, it then disappears, and you face the challenge of coming up with some even more ingenious solution.

Now we are some way into the middle stages of Alzheimer's with one parent, and at the early stages with the other, we have developed some kind of perspective on the situation. Even though this perspective seems tenuous, and liable to fall apart at the next turn of events, some aspects of it are fairly stable, and appear likely to remain that way.

I don't resent my parents turning out how they have; far from it. I can see that I have quite a significant debt of care (given to me during my childhood) to repay, and in a world where so many of our activities are frivolous and ultimately meaningless, it is satisfying to be engaged in such an inherently meaningful activity. A few years ago I got chatting to a Chinese gentleman sitting next to me on an aeroplane. Our discussion became quite philosophical, and centred on values and meaning. He told me that I must value my parents, otherwise one day they would be gone and I would be left with regrets for the rest of my life. I suppose that at the time I appreciated this only in an academic sense; the contingency did not seem very immediate. Now, however, the question of value is clearly coupled with an answer. Our behaviour towards mum and dad these days is hard evidence of the value we place on them. When they do eventually die, I doubt very much that we will feel that we didn't do enough for them. Others may feel that, but I doubt we will.

So there is a kind of light at the end of the tunnel, after all.

So, yes, we value them, but how much? For a while, I looked forward to guiding my parents through their later years. I may have been even more naive than I now think; our parents are so physically healthy that they could easily both live - and steadily decline - for another ten years. Am I truly prepared to look after them for that long, each year more demanding than the one before? I really cannot say.

One way or another, Alzheimer's is opening a new chapter in all our lives, one that will probably test our character more than any other.