Scientific giants that they were, Albert Einstein and Sigmund Freud rarely occupied the same intellectual space.

Yet these superheroes of 20th-century science, argues author Richard Panek, are linked by the way they both marshaled scientific evidence - invisibly. Each moved beyond the perceived limits of the science of their day, not by looking farther into the cosmos or deeper into the human mind, but by thinking about problems in a whole new way. Both men thus encountered unexpected insights in the realm of the unseen.

For Einstein, the invisible came as a mental melding of space and time. Since the days of Galileo, astronomers had progressed by building ever-larger telescopes, each technological advance allowing them to peer farther into the universe. But Einstein pushed forward with absolutely no telescope at all. His greatest breakthrough - formulating the general theory of relativity that would revolutionize physics - arrived entirely in his mind as he pictured how two bodies in motion would appear to each other.

For Freud, insight arrived as the subconscious. In an era when neuroanatomists struggled to find a brain-based reason for every psychological complaint, Freud moved beyond dissection. Instead of hypnotizing patients, as was common at the time, he found greater success in coaxing memories to arise on their own. There, just beneath the surface, he discovered the suppressed subconscious.

Panek doesn't simply discourse on the lives of Einstein and Freud; biographical details give way to richer philosophical musings. In the course of 200 terse, elegantly written pages, the book metamorphoses from a primer on two famous researchers into a discourse on the history of science.

In Panek's hands, science retains its historical status as "natural philosophy" - a way of wondering about and investigating the world, and a far cry from the bow-tied, lab-coated stereotypes of modern research.

"The Invisible Century: Einstein, Freud, and the Search for Hidden Universes" by Richard Panek; Viking ($24.95)