Chris Wallace had a great love of speculative argument, especially the exploration of 'what if' scenarios. In idle times, he would indulge in this often at great length.

There was one time when I started a discussion on the use of air independent propulsion systems for submarines, using fuel cell technology. Chris observed that the German navy experimented with hydrogen peroxide as a fuel for submarine engines, and we spent a whole hour arguing the virtues and design issues involved.

On another occasion, we argued about the two competing theories of the demise of the Neanderthal man, an issue which remains hotly contested amongst many scholars of human evolution. Being the student of human behaviour Chris was, he did observe that if Homo Sapiens and Neanderthal man had the biological capacity to interbreed, then man's history suggests that would have been exactly what did occur.

Many times Chris argued about the findings of the famous twins studies, in which biologically identical twins separated at birth were examined decades later with the astonishing finding that a great many of them shared even trivial details in their lives, down to preferences for colours and types of cars. The statistical evidence of biological determinism in individual development found by this study was something Chris found intriguing.

One time we argued the merits or otherwise of dirigible airships, a mode of locomotion which vanished with the so public demise of the Hindenberg over New York. Chris had a fondness for dirigibles, and we expended many hours discussing the characteristics of the British and German designs of the 1920s and 1930s. This discussion restarted years later after a television documentary was aired about the Hindenberg accident and the pros and cons of the use of hydrogen lift gas in dirigible designs were debated at length. Ever the pragmatist, Chris saw value in the idea of reviving dirigibles as a technology in daily use. That tethered dirigibles are now seeing a renaissance as vehicles for carrying communications equipment and radars is perhaps the ultimate proof of Chris' insight.

While Chris was a tough and sceptical scientist, this did not deter him from arguing about the truly esoteric if not unconventional. At one time we indulged in debating the what ifs surrounding the existence or non-existence of advanced extra-terrestrial species. I observed that it would be good fun to pull apart a hypothetical alien space vehicle to learn how it worked. Chris observed that this would be a guaranteed failure, no differently from a scientist of 1945 trying to divine the secrets of modern microprocessor chip!

Chris was fascinated with the physics of propulsion systems, especially rocketry and advanced schemes for generating thrust. This was the topic of many a technical argument.

His fascination for the esoteric was not confined to biology, aviation, propulsion and space exploration. At one time we argued at much length about the possibilities in constructing non-electrical computing machines. While the Babbage engine remains the archetypal case study, Chris observed that a smarter strategy for the technology of that era would have been to raid the pneumatic switch and actuator technology of that period, by then highly refined by the pipe organ industry. Within that hour, Chris outlined a design for a primitive central processing unit powered by compressed air. Indeed had history taken a different turn, the first generation of computing engines might have indeed been driven by a steam engine powered compressor.

Chris was always uncomfortable with man's hubris, perhaps not surprising given his immense personal humility. He recognised that much of what the modern world perceives to be new is actually very old, if not ancient. Of all of the papers I produced over the last decade, the one Chris enjoyed perhaps the most was a study of biological examples of information warfare, and its ancient evolutionary origins as a survival mechanism. Perhaps symptomatic of what Chris found so disturbing about modern man's hubris, one reviewer refused to accept that information warfare could be anything but man-made.

Very often Chris extolled the virtues of the Russian approach to designing space equipment, centred in using as much proven technology as possible. The example he found of most interest was the use of fire engine water pump components in early Soviet turbopumps. It was this example which compelled Chris to use kitchen hood fans for cooling the multiprocessor prototype, in preference to much more expensive and less reliable computer fans. Suffice to say, the cheap and nasty kitchen fans lasted a lot longer than any computer fan, then costing several times as much.

Thinking about choosing simple and reliable solutions reflects a deeper philosophical perspective held by Chris. His work on Minimum Message Length theory, an embodiment of Occam's philosophy, reflects a much broader and deeper fascination with the idea of simple and elegant solutions to complex problems.

One of his favourite approaches when tackling a new problem was to first ask 'what does nature permit here?' If nature permitted something, then we could explore further and try to divine the solution. But always, Chris sought to strip away the man-made clutter surrounding a problem before extracting its essence, in its simplest form.

Chris had an insatiable curiosity about everything ever created or learned, no matter what the area might have been. He once observed that he had a deep interest in understanding how the world worked, which is what compelled him to start work on the MML project. The vast repository of ideas and knowledge he collected and generously shared over the decades underpinned much of his hard and critical analysis of new problems. It provided him with a library of examples against which almost any hypothesis could be tested for disproving counterexamples. If none were found, then perhaps it was something which nature permits, worthy of further exploration.