Final answer:
Isaac Newton was mostly self-motivated in his young years, particularly during the plague years when he was working alone, but Edmund Halley played a significant role by encouraging Newton to publish his work. The inspiration from observing an apple fall, as well as the earlier groundwork laid by scientists like Galileo and Kepler, also contributed to Newton's intellectual development.
Step-by-step explanation:
While there isn't a single individual who can be credited with being the most significant motivator for Sir Isaac Newton when he was young, it's clear that his own curiosity and observations of the natural world played a pivotal role. The famous story of Newton's apple suggests that the simple act of watching an apple fall from a tree may have inspired some of his most notable ideas about gravitation. Additionally, Edmund Halley played a critical role in Newton's life; not only did Halley encourage him to publish his ideas, but he also financed the printing of Newton's groundbreaking work, 'Philosophiae Naturalis Principia Mathematica'.
Newton's progression as a thinker was also implicitly influenced by the work of his predecessors such as Galileo Galilei, Johannes Kepler, and Tycho Brahe, who laid the groundwork for his discoveries. Moreover, upon sharing his work, the likes of Émilie du Châtelet were instrumental in championing and disseminating Newtonian physics across Europe.
Ultimately, it is the amalgamation of Newton's natural philosophy interests, personal determination, and the support of influential figures like Halley and Du Châtelet that catalyzed his development into one of history's most revered scientists.