The body has an effective system of natural defence against parasites, called the immune system. The immune system is so complicated that it would take a whole book to explain it. Briefly, when it senses a dangerous parasite, the body is mobilized to produce special cells, which are carried by the blood into battle like a kind of army. Usually the immune system wins, and the person recovers. After that, the immune system remembers the molecular equipment that it developed for that particular battle, and any following infection by the same kind of parasite is beaten off so quickly that we don't notice it. As a result, the weakened immune system leads to infection, and the infection causes damage to the immune system, which further weakens resistance. That is why, once you have had a disease like the measles or chicken pox, you're unlikely to get it again.

Today's "digital natives" have grown up immersed in digital technologies and possess the technical aptitude to utilize the powers of their devices fully. But although they know which apps to use or which websites to visit, they do not necessarily understand the workings behind the touch screen. People need technological literacy if they are to understand machines' mechanics and uses. In much the same way as factory workers a hundred years ago needed to understand the basic structures of engines, we need to understand the elemental principles behind our devices. The lifespan of devices depends on the quality of software operating them as well as the structure of hardware. This empowers us to deploy software and hardware to their fullest utility, maximizing our powers to achieve and create.

Inflationary risk refers to uncertainty regarding the future real value of one's investments. Say, for instance, that you hold $100 in a bank account that has no fees and accrues no interest. If left untouched there will always be $100 in that bank account. If you keep that money in the bank for a year, during which inflation is 100 percent, you've still got $100. Only now, if you take it out and put it in your wallet, you'll only be able to purchase half the goods you could have bought a year ago. In other words, if inflation increases faster than the amount of interest you are earning, this will decrease the purchasing power of your investments over time. It would be very useful to know in advance what would happen to your firm's total revenue if you increased your product's price. That's why we differentiate between nominal value and real value.

Academics, politicians, marketers and others have in the past debated whether or not it is ethically correct to market products and services directly to young consumers. This is also a dilemma for psychologists who have questioned whether they ought to help advertisers manipulate children into purchasing more products they have seen advertised. Advertisers have admitted to taking advantage of the fact that it is easy to make children feel that they are losers if they do not own the ‘right' products. When products become more popular, more competitors enter the marketplace and marketers lower their marketing costs to remain competitive. Clever advertising informs children that they will be viewed by their peers in an unfavorable way if they do not have the products that are advertised, thereby playing on their emotional vulnerabilities. The constant feelings of inadequateness created by advertising have been suggested to contribute to children becoming fixated with instant gratification and beliefs that material possessions are important.

In peer-reviewed publications, scholars who are at arm's length from one another evaluate a new experiment, report, theory, or claim. They must be expert in the domain they're evaluating. The method is far from foolproof, and peer-reviewed findings are sometimes overturned, or papers retracted. Peer review is not the only system to rely on, but it provides a good foundation in helping us to draw our own conclusions, and like democracy, it's the best such system we have. If something appears in Nature, The Lancet, or Cell, for example, you can be sure it went through rigorous peer review. As when trying to decide whether to trust a tabloid or a serious news organization, the odds are better that a paper published in a peer reviewed journal is correct.

Many animals live a life that I would just as soon forgo, not because it ends in tragedy, but because their approach to living is one of endless conflict. Imagine being a seagull and spending your entire life fighting other seagulls for scraps. What makes us so lucky is the pure happenstance that we evolved to be (mostly) good to one another. Our cooperative nature also set the stage for the evolution of our amazing brain. Our sociality made us smarter individually, but, far more important, it connected our minds to others' minds in a manner that massively increased our knowledge and computing power. As a result, we long ago surpassed the predators that hunted us on the savannah, and are now holding most of the pathogens at bay that are a much greater threat than predators ever were. For the first time in history, we no longer bury almost half our children before they reach adulthood. Evolution is brutal, but those of us with the good fortune to live in established democracies have used the tools that evolution gave us to create unprecedentedly safe and satisfying lives.

Strengths are contextual. Any personal quality can be either an aptitude or a handicap, depending on the situation. Let's say you have difficulty reading printed text. This might understandably seem like a shortcoming, especially if you want to be a literary critic, a profession heavily dependent on the parsing of texts. But if you want to be an astronomer, the same apparent shortcoming could turn into an unexpected strength. The brains of many people who have trouble reading are better at detecting black holes and other celestial anomalies in astronomical images than the brains of individuals without reading difficulties. A facility for empathy is an asset for a nurse, but a shortcoming for a military drone pilot. Being tall is an advantage for an NBA player, but a disadvantage for a coal miner.

Our ideas can liberate or imprison us. In a literal sense we create the worlds in which we live; and there is always the possibility of re-creation. As psychologist George A. Kelly put it: "to make sense out of events we thread them through with ideas and to make sense of the ideas we must test them against events." He describes this process as one of successive approximations. The great generative ideas in human history have transformed the world view of their times and helped to reshape their cultures. We make the world we live in and we can remake it. This process of cultural evolution is probably what the comedian George Carlin had in mind when he said, "Just when I found out the meaning of life, they changed it." What is true of the long cycles of creative change in a social culture is also true of the shorter cycles of creative work by individuals and groups.

The secrets of human curiosity have been explored by psychologists, perhaps most famously by Professor George Loewenstein. He writes of a test in which participants were confronted by a grid of squares on a computer screen. They were asked to click five of them. Some participants found that, with each click, another picture of an animal appeared. But a second group saw small component parts of a single animal. With each square they clicked, another part of a greater picture was revealed. This second group were much more likely to keep on clicking squares after the required five, and then keep going until enough of them had been turned that the mystery of the animal's identity had been solved. Brains, concluded the researchers, seem to become spontaneously curious when presented with an 'information set' that they realise is incomplete. 'There is a natural inclination to resolve information gaps,' wrote Loewenstein, 'even for questions of no importance.'

Wood was such an essential component of everyday life that one might expect a limit to wood production was responsible for the lack of progress. Almost all the possessions of everyday folk were wooden, while those that were not actually made of wood needed large quantities of wood to produce. In the Middle Ages, around thirty pounds of wood were needed to smelt one pound of iron, for instance. People burned wood in even greater quantities to cook food and heat their houses, and wood was a vital energy source for the major industrial processes of the age: salt-making, brewing, tanning, and dyeing. In the language of chess, wood was an overworked piece. And as the human population grew, and land was cleared for agriculture, forests would have been destroyed, reducing the wood supply still further. You might well think that this would eventually have led to a shortage of wood, hindering further material progress. Popular histories, after all, are full of stories of how using wood led to deforestation and disaster.