Fundamental, basic, basal, underlying, radical are comparable when they mean forming or affecting the groundwork, roots, or lowest part of something.
Fundamental is used chiefly in reference to immaterial things or to abstractions, whether they are thought of as built up on a foundation or as having their origins in roots.
Basic is often used interchangeably with fundamental when the latter implies reference to a substructure. But basic is preferred to fundamental when the reference is to a definite or concrete groundwork, bottom, or starting point.
Basal differs from basic chiefly in not being used as often in reference to immaterial things and in more often implying reference to the bottom or to the lowest point or regions of a thing.
Underlying may be used to suggest nothing more than extension beneath something else. However, especially when the reference is to something immaterial, the term frequently comes close to fundamental, differing from it chiefly in suggesting a depth that removes the thing from one’s range of vision or a remoteness that demands study or research on the part of one who would detect it.
Radical (see also LIBERAL 2 ) implies reference to the root or origin or ultimate source of a thing; thus, a radical change is one that is so thoroughgoing that it affects the fundamental character of the thing involved; a radical error touches the very center and source of a thing’s life.