This, Carl Sagan's last book, is a characteristically wide-ranging mix of science and compassion. It looks at

• Abortion; with wife Ann Druyan, he builds a persuasive argument based on the assertion that a child may be considered sentient from around the 6^th month, when, he says, the large-scale linking-up and coordinated interaction of brain cells begins to take place.

• Environmental protection and global warming. He notes that the difference between glacial and interglacial periods is about 3-6 degrees celsius, and sets this against the predictions by most knowledgeable institutions that if CO² levels continue increasing at the present rate, the climate will probably warm by between 1 and 4 degrees celsius. (These changes are extremely difficult to calculate).

Although scaremongering is something which Sagan avoids, he refers to an alarming suggestion by Wallace Broecker of Columbia University that it is possible for there to be a threshold beyond which a warming climate (such as Earth's) makes a transition to a completely different stable state. In other words, the atmosphere can deal with changes up to a certain level; but it is conceivable that beyond a certain point, the atmosphere may begin to deal with heat in different ways, and for the frequently-mentioned 'runaway greenhouse effect' (or an equivalent) to actually take place, and for the surface of the Earth to become an insufferably hot place. Of course, this is only a possibility; we don't know all that much about how our hugely complex atmosphere works under present conditions, let alone under warmer ones.

Although he was an agnostic, he was unblinkered enough to see the opportunity to harness the social power of religious organizations, and wrote a message to them asking for their help in campaigning on environmental issues: "Science and religion may differ about how the Earth was made, but we can agree that protecting it merits our profound attention and loving care." This lack of tribalism was very much a Sagan trademark.

• Life and intelligence in the Universe (and in particular the fairly recent and so far inconclusive evidence for life on Mars).

• A plea for critical thinking. He quotes Rudolf Hess as an example of how not to think. In 1934, Hess said that

"One man remains beyond all criticism, and that is the Fuhrer. This is because everyone senses and knows He is always right, and he will always be right. The National Socialism of all of us is anchored in uncritical loyalty, in a surrender to the Fuhrer".

• The dangers posed by nuclear weapons, jingoism and powerful nations who throw their weight around. He remarks on the peculiar untouchability of the US's defence budget even in this post-Cold War era, and recounts with customary even-handedness the abuses committed by the US and the Soviet Union;

"Excluding World Wars and expeditions to suppress piracy or the slave trade, the United States has made armed invasions and interventions in other countries on more than 130 separate occasions, including China (on 18 separate occasions), Mexico (13), Nicaragua and Panama (9 each), Honduras (7), Colombia and Turkey (6 each) . . . " [the list continues].

It's great to come across a mind as questing, polymathic and humane as Sagan's.

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Carl Sagan's books are as readable as they come; the man's enthusiasm, erudition and mental scope were such that it is not surprising he became the most popular science writer ever (this title may since have been wrested from him by Stephen Hawking with his inaccessible 'A Brief History of Time'). 'Cosmos', the accompaniment to the smash-hit TV series, is the most popular of his books. True to form, it is eclectic and unflaggingly interesting; it trawls through a huge range of subjects, among them the history of astronomy, the Big Bang, the origin of life, evolution, extraterrestrial life and SETI, cometary impacts, panspermia, the exploration and colonization of space, stellar life cycles, and whalesong. Great stuff.

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'Pale Blue Dot' is something of an update of 'Cosmos'. However, it is no tired sequel; Sagan's enthusiasm shines through as strongly as ever. He relates many of the interesting scientific developments - in particular, exploration of the outer solar system - which have taken place since 'Cosmos' was written. The book places more emphasis on humanity's move into space - a project which Sagan promoted with gusto.

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