How did Gottfried Wilhelm Leibniz describe monads?

In his work Monadologia (1714), Gottfried Wilhelm Leibniz described monads as the basic, immaterial substances that constitute the universe. Because they lack spatial extension, they are not physical objects but rather elementary, individual entities.1

Leibniz characterized monads through the following attributes:

• Nature: They are unique, indestructible, and dynamic, soullike entities.1
• Function: Their properties are determined by their perceptions and appetites.1
• Causality: Monads do not have true causal relations with one another. Instead, their apparent interactions are explained by preestablished harmony, a state in which God synchronized all monads at the beginning of time so that they perfectly reflect one another’s changes.12
• Material World: The objects we perceive in the material world are simply appearances resulting from collections of these monads.1

Leibniz’s concept evolved from his early interest in the existential value of the individual and his later realization that time, space, and motion could not be considered fundamental substances.3