The Cosmological Turn

There are some turns from which there is no turning back. Approaching such a turn, one is not of course compelled to press on; the options of retreat or diversion are always available, but only at the cost of consigning oneself to the familiar, well-trodden surroundings. If, however, eschewing such provincialism, one makes the turn and heads across the hitherto unknown landscape, there is no turning back. Never again will things be quite the same.

To be sure, the turn that has now become imperative cannot but reiterate in some measure the turn to nature that philosophy has repeatedly been compelled to carry out: the turn from the polis to the natural universe that occurs when the voice of Timaeus displaces that of Socrates; the turn from society and its inscriptions back to the pure state of nature in search of the origin of human inequality; the turn entailed by the declaration that the whole of modern European philosophy has as its common defect that nature does not exist for it; and finally, the imperative that those to come remain true to the earth. Yet the imperative that imposes itself today demands a turn for which none of these precedents provides a sufficient measure; it requires a turn that would be both more encompassing and more disruptive in its recoil on philosophical thinking. Let us call it the cosmological turn.

There are occasions—rare perhaps, but provocative—when something never seen before comes to be seen. There are also occasions when something previously seen only vaguely and indeterminately—a star, for instance, that appears as a mere shimmering point of light—comes to be seen with a clarity and extension sufficient to allow its determination. Such previously unseen or barely seen sights may call for hitherto unthought thoughts. In the most provocative instances, they prompt a turn from which there is no turning back.

In 1572 Tycho Brahe observed the sudden appearance of a new star in the constellation Cassiopeia. The star was so bright that at its peak luminosity it was visible even in daylight. Tycho determined the exact position of the star and compiled a record of its changing brightness from the time of its appearance until, five months later, it faded entirely from view. By means of parallax he was able to demonstrate that the distance from the earth to the star was greater than that from the earth to the moon. In other words, the star was shown not to be sublunary.

In 1609 Galileo turned the newly invented telescope heavenward. By means of this instrument he was able to confirm something that was only vaguely evident to unaided perception: that the surface of the moon is not smooth and polished but, rather, as Galileo wrote in his treatise Sidereal Messenger, is “full of enormous swellings, deep chasms, and sinuosities.” “But,” he continues, “what by far surpasses all admiration, and what in the first place moved me to present it to the attention of astronomers and philosophers, is this: namely, that we have discovered four planets, neither known nor observed by anyone before us, which have their periods around a certain big star previously known.”¹ The “big star” was of course the planet Jupiter, and the “four planets” were the moons of Jupiter.

On October 4, 1923, Edwin Hubble focused the 100-inch reflector telescope at the Mount Wilson Observatory on one of the spiral arms of M31, known at that time as the Andromeda Nebula. The telescope was fitted with a camera that allowed long-term exposure of the photographic plate to the incoming light. Hubble took a forty-minute exposure and then on the following night repeated the procedure, increasing the exposure time by five minutes. When he examined the plates and compared them with earlier photographic examples, he determined that one of the objects photographed was a type of bright, pulsating star called a Cepheid. Such stars vary regularly in their brightness, becoming dimmer and brighter with periods from a few days to a few months; this first Cepheid that Hubble found in M31 had a period of 31.4...