[4] When conservative Paul IV became pope in 1555, he instituted a rigorous campaign to suppress heresy in the Italian States.

[3] At the same time he published a secular work, De Methodo, hoc est, de recte investigandarum tradendarumque Scientiarum ratione, which lays out an approach to intellectual inquiry that emphasizes a rational, almost mathematical approach, proceeding from clear and concrete first principles.

[4] While in Switzerland, Aconcio became acquainted with some of the Marian exiles, English Protestants who had fled persecution under the reign of Queen Mary.

In 1563 Aconcio proposed a plan to drain two thousand acres of marshland along the south bank of the Thames between Erith and Plumstead.

The effort was initially successful, but bad weather flooded what he had managed to reclaim, and Aconcio was obliged to hand over control of the project to his partner, Giovanni Battista Castiglione, and other investors.

[4] It was not until 1564 that he was hired as a military engineer to participate in a joint review of the fortifications being developed at Berwick Castle, one of the most important strongholds along the Scottish-English border.

The work being done by engineer Richard Lee had been criticized by the government's chief Italian expert, Giovanni Portinari, and Aconcio was sent to Berwick to provide another perspective.

[citation needed] The dedication of such a work to Queen Elizabeth illustrates the tolerance or religious laxity during the early years of her reign.