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Can someone tell me what is the opposite of a Boltzmann brain (29 March, p 46) ? It seems to be a self-reproducing brain that arises through a copy-construction process, not spontaneously from thermal noise. But the complexity of a zygote is many orders of magnitude lower than that of a brain. So random thermal …

• We stated that there have probably been fewer than 25,000 generations since the human lineage split from that of chimpanzees (19 April, p 30) . As several readers have spotted, there was a missing zero: it should have said 250,000. • In the graph accompanying our evolution special (19 April, p 33) , the …

Thank you for the balanced debate on the important issue of "feeding the world" (15 March, p 5) . It will probably elicit the old objections that those opposed to genetically modified crops are "anti-science". This was always an unfair response. We have long been missing a critical discussion of what the technical potential of …

I am gratified that Deborah Keith thinks that the plant science industry cares passionately about abating hunger and supporting rural development (5 April, p 17) . But development into what? Towards a situation where even small, Third World farmers who have been growing traditional crops developed over generations to suit local conditions are forced to …

I was interested by Phil McKenna's description of the problems of voting systems (12 April, p 30) , but disappointed that the suggested alternatives did not allow the electorate to do what I propose they really want: to vote against a candidate. Talking to people about their voting intentions always brings up the worry that …

I would be interested to know whether John Denham still believes that the UK government – of which he is a member – can deliver "a properly organised identity card system, as long as it's properly structured, as long as the data is held in the right way and so on" (5 April, p 44) …

Your editorial on forgetting the nuclear past (12 April, p 5) mentions as the worst nuclear accident in US history the one at Three Mile Island, Pennsylvania, in 1979. I was about 150 kilometres to the north at Penn State University at the time, but that one didn't bother me. What did was the much …

It seems odd to try to come up with weird and wonderful methods to "pull" carbon dioxide out of the atmosphere ( 29 March, p 36 , for example). We already have extremely efficient, tried and tested, solar-powered devices that can easily be produced in large numbers with little investment. They are called plants, and …

You refer to the "ecosystem services" of rainforests as having "a commercial value of zero" (5 April, p 5) . I understand that we derive about half our drugs and medicines from the gene pool of the world's flora and fauna, but that we have researched only about one-tenth of the species in the ecosystems …

I can accept that Deborah Keith did, as she says, choose a career in crop science in order to combat world hunger (5 April, p 17) . But the managers of the company she works for have only one goal: to maximise profits. To claim it is guided by a philanthropic mission statement is at …

Your feature on the health benefits of calorie reduction in Cuba after the collapse of the Soviet bloc (12 April, p 6) fails to consider a key variable. The Cuban food supply, as well as being restricted, also became organic. You have reported a link between type 2 diabetes and pesticide residues in food (14 …

Scientism has come to signify the view that only those fields of inquiry that employ the methods and explanatory styles of the natural sciences count as generating genuine knowledge and genuine explanations (12 April, p 48) . This is absurd because it would imply that most ordinary, everyday and quite proper claims to knowledge and …

One key sentence in your evolution special (19 April, p 24) gave me cause for concern. Michael Le Page wrote: "The success of western civilisation is based on science and technology, on understanding and manipulating the world." There is plenty of evidence that this endeavour is failing. We are now all aware that western civilisation …