Why did Charles II of Spain choose Philip duc d’Anjou as his heir?

Charles II of Spain chose Philip, duc d’Anjou—the grandson of Louis XIV—as his heir in his final will, dated October 1700, primarily to ensure the integrity of the Spanish Empire.1

Following the death of the previous heir presumptive, the electoral prince Joseph Ferdinand, in 1699, Charles II sought to prevent the partitioning of his territories, which had been proposed by France, England, and the United Provinces in earlier treaties. By naming Philip as his successor, Charles II mandated that the Spanish dominions remain undivided and be kept separate from any other crown.1

This decision was a strategic attempt to preserve the Spanish inheritance against the competing claims of the House of Habsburg, specifically the Archduke Charles, while avoiding the fragmentation of the empire that would have resulted from the partition treaties.12