Natural, simple, ingenuous, naïve, unsophisticated, artless, unaffected are applied to persons, their acts, and their utterances, in the sense of wholly free from pretension or calculation.
Natural implies, on the one hand, freedom from every sign of artificiality, effort, constraint, or affectation and, on the other hand, an ease, a spontaneousness, or a flexibility that suggests nature rather than art; the term often implies opposition to whatever is labored, stiff, formal, or artificial.
Simple stresses complete freedom from everything that might suggest unconscious or conscious duplicity. It usually implies lack of confusion of aims, desires, interests, or values and therefore may carry one or the other of connotations as divergent as mental immaturity and intellectual ripeness, as the lack of experience characteristic of the child and the fullness of wisdom characteristic of the sage, or as the transparency of those who do not know how to conceal their nature or motives and that of those who have nothing to conceal.
Ingenuous stresses inability to disguise or to conceal one's thoughts or feelings; it usually implies frankness or candor, lack of reserve, or freedom from dissimulation, often with a hint of childlike simplicity.
Naïve implies freedom from all that is artificial, conventional, or acquired; in early use and still often, especially in its derivative noun naïveté, it suggests freshness, spontaneity, and genuine expression of a nature untouched by worldly influences and without affectation or artifices.
But it may sometimes become a term of derogation and then often implies lack of worldly wisdom. Equally often, especially in learned use, it suggests the point of view of the untutored or unenlightened person or of one whose judgments are not corrected by advanced scientific or philosophical knowledge and who therefore supposes that things are what they seem to be.
Unsophisticated also stresses lack of wisdom, especially worldly wisdom. It does not, however, emphasize native simplicity as strongly as naïve; rather, it suggests lack of the experience or training necessary for worldly success or, more specifically, for graceful and adroit social relations.
Artless lays the stress on the absence of design; it suggests naturalness that is the result of indifference to, or unawareness of, the effect or impression one is producing.
Unaffected centers the attention on the absence of affectation, but it usually implies both naturalness and simplicity without any hint of childishness, unworldliness, guilelessness, or indifference.