continues to view him as Western culture's primary example of a "spaceman," that is, as a genius so transcendent that he could reach, in his own fifteenth century, conclusions that the rest of science, plodding forward in its linear march to truth, would not ascertain for several hundred years. Leonardo stood alone and above, we are told over and over again, because he combined his unparalleled genius with a thoroughly modern methodology based on close observation and clever experiment. He could therefore overcome the ignorance and lingering sterile scholasticism of his own times.

For example, the "Introductory Note" in the official catalog for a recent exhibition of the Leicester Codex in New York summarizes the basis of Leonardo's success in these words; "In it [the codex] we can begin to see how he combined almost superhuman powers of observation with an understanding of the importance of experimentation. The results were inspired insights into the workings of nature that match his artistic achievements." When such conventional sources acknowledge the persisting medieval character of many Leonardian pronouncements, they almost always view this context as a pure impediment to be overcome by observation and experiment, not as a matrix that might have been useful to Leonardo, or might help us to understand his beliefs and conclusions. For example, the closing passage of the long Encyclopaedia Britannica article on Leonardo states: "Leonardo approached this vast realm of nature to probe its secrets ... The knowledge thus won was still bound up with medieval Scholastic conceptions, but the results of his research were among the first great achievements of the thinking of the new age because they were based on the principle of experience."

Frankly, and to say so quite badly, I think that this conventional view could not be more wrong in its general approach to the history of knowledge, or more stultifying for our quest to understand this most fascinating man of our intellectual past. Leonardo did make wonderful observations. He did often anticipate conclusions that public science would not reach for another two or three centuries. But he was neither a spaceman nor an angel-and we will never understand him if we insist on reading him as Hank Morgan, a man truly out of time, a modernist among the Medici, a futurist in the court of Francis the First.

Leonardo operated in the context of his time. He used his basically medieval and Renaissance concept of the universe to pose the great questions, and to organize the subjects and phenomena, that would generate his phenomenal originality. If we do not chronicle, and respect, the medieval sources and character of Leonardo's thought, we will never understand him or truly appreciate his transforming ideas. All great science, indeed all fruitful thinking, must occur in a social and intellectual context-and contexts are just as likely to promote insight as to constrain thought. History does not unfold along a line of progress, and the past was not just a bad old time to be superseded and rejected for its inevitable antiquity.

In this essay, I will try to illustrate the centrality of Leonardo's largely medieval context by analyzing his remarkable paleontological observations in the Leicester Codex. I will begin by acknowledging their truly prescient character, but will then raise two questions that expose the early-sixteenthcentury context of Leonardo's inquiry: first, "What alternative account of fossils was Leonardo trying to disprove by making his observations?" and, second, "What theory of the earth was Leonardo trying to support with his findings?" Leonardo did not make his observations to win the praises of future generations; he studied fossils to probe these two questions of his own time-and his answers could not be more deeply embedded in a "hot topic" of his own century that we would now mock and dismiss as hopelessly antiquated. Thus, we cannot understand Leonardo's paleontology when we only marvel at his empirical accuracy and ignore the reasons for his inquiry.

Yes indeed, a thousand times yes, Leonardo's observations are often stunningly accurate-as experts have always said, and for the reasons conventionally stated. Moreover, their degree of detail, and their centrality to the basic rules of modern paleoecological analysis, only enhance the impression of authorship by a Victorian geologist somehow trapped in the early sixteenth century. But let me stop marveling and start listing a small sample!

1. Leonardo recognized the temporal and historical nature of horizontal strata by correlating the same layers across the two sides of river valleys:

How the rivers have all sawn through and divided the members of the great Alps one from another; and this is revealed by the arrangement of the stratified rocks, in which from the summit of the mountain down to the river one sees the strata on the one side of the river corresponding with those on the other.

(All quotes, unless otherwise stated, come from the Leicester Codex as presented in the MacCurdy translation of Leonardo's notebooks.)

2. He observed that rivers deposit large, angular rocks near their sources in high mountains, and that transported blocks are progressively worn down in size, and rounded in shape, until sluggish rivers deposit gravel, and eventually fine clay, near their mouths. (I learned this rule as principle number one on day number one in my college course in beginning geology.)