The antonyms for the word "concrete ideas" would be "abstract concepts" or "vague notions." These terms describe ideas that are not specific or tangible, but rather general or theoretical in nature. Abstract concepts refer to ideas that are difficult to define or explain, while vague notions are ideas that lack clarity or specificity. For instance, while "love" is an abstract concept as it cannot be touched, it is also a concrete idea as its meaning is more defined. It is important to be able to communicate effectively in both concrete and abstract terms in order to have a well-rounded understanding of any given subject.