The first is the difficulty of dissipating heat without the cooling (vascular) systems dominating the brain space - we generate about the same heat output as a 50W light bulb in a 10cm cavity.

Second, the signal transmission span and synaptic speed are dictated by reaction times essential to avoid physical threats and danger. There are also limiting trade-offs between the size of the vascular system and the density, capacity and synaptic interconnection. Intelligence depends on rapid and massive signal processing of pulses at the synapses. In computer terms the pulse width is linked to the clock speed, and processing cannot occur faster than a single clock cycle. If pulses cannot be made shorter then they dictate the maximum processing speed.

For our metabolism to support a bigger brain would require more blood flow, faster food-to-energy conversion, and a stronger neck. A much larger brain would need more damping for shock absorption. With a 1m head we would be in danger of concussion every time we started or stopped walking as our brain crashed into the inside of our skull.

Whales and dolphins have larger brains, but with large portions devoted to sonar processing and communication over long distances. They also enjoy a larger vascular system and a cooler environment.

While the measurable differences in size, and perhaps structure, of male and female brains has become a high point of "political correctness", size/intelligence estimates show little difference and may even favour the smaller brain as having marginally better packing density and connectivity.

But this difference appears to be related to the need to throw projectiles accurately, with thousands of our neurons acting in parallel to overcome their individual noisy nature. Unlike silicon brains, ours deals in uncertainty, with the biological neurons constructed from about 1,000 cells. Such a small number makes them essentially noisy - in some ways, quantum devices.

In stark contrast, we design transistorised logic to switch with great precision. One world is full of life, emotion and intelligence; the other is cold, deterministic and dead. This realisation has recently seen the engineering of noisy neural networks purposely arranged not to be deterministic.

So what separates us and our noisy neurons from those in the latest machines? Only scale and sensors. Our awareness comes from sight, sound, touch, smell and taste. We can now give all of this to a machine in a form that could be superior to ours. They could have experience over a far wider spectrum than us, and also have additional abilities. It would thus seem that the only limiter is scale. Given the present rate of progress, the year 2015 may see us equalled.

What price political correctness when we have man, woman and machine? It might just be that machine born of man will be smartest of all.

Peter Cochrane holds the Collier Chair for the Public Understanding of Science & Technology at the University of Bristol. His home page is:
http://cochrane.org.uk