In this phrase, Washington is emphasizing the importance of practical application over theoretical knowledge. He argues that simply filling someone's mind with abstract concepts and information—without tying that knowledge to real-world situations and experiences—has little value.
Washington suggests that education should not only focus on intellectual development ("mental training") but should also include moral and ethical considerations ("moral training"). The ideal of education, in his view, is not just to produce individuals who can excel in academic settings, but rather to develop well-rounded individuals who can apply their knowledge effectively in practical situations and contribute positively to society.
By critiquing "mere abstract knowledge," Washington advocates for a more holistic approach to education that involves experiential learning, vocational training, and a strong moral foundation. This reflects his belief that education should empower individuals to engage meaningfully with their communities and the world around them. In essence, he is calling for an education that is relevant, grounded in reality, and capable of fostering both personal growth and societal progress.