I can not agree with the speaker that people's behavior is largely defined by the external forces. As a creature living in the human society, people's behavior cannot be avoid affections from both the external and internal. I admit that external factors have it power to affect people's behavior to a large extent, but internal factors have the real power to determine how people behave.Let us imagine that people's behavior is indeed largely determined by external forces, what the society would be like. Giving up their self-awareness, people  will deteriorate into a machine with no sense of responsibility and bring about problems in the society. Firstly, a student driven by others has a high possibility of cheating in the exams which is immoral.  By making education disorder, externally driven behavior will lead to low-quality education. Secondly,  a worker who let the outlying environment affect himself can not make it a reliable one. Deflecting working responsibilities and undermining team work is about to result in a underperformed productivity. Besides, a child who always flee of his responsibility would end up a unfilial one who will also not make any contribution to the development of society. Based on what we discussed above, we can draw a conclusion that letting external factors determine people's behavior could inevitably weaken the progress of human society.In contrast, people who have strong self-awareness and sense of responsibility could bring the society with a bunch of benefits. Students follow the examination rules and do not commit disruptive behavior s; Workers undertake their own task respectively in a collaborative manner, productivity thus is improved significantly; Children with filial piety make their contribution to families,  getting along with neighbors harmoniously. In one word, being responsible to his own behavior is being to responsible to the society. As we know, everyone's behavior is under the influences of both external causes and internal causes. Philosopher Karl Marx's materialistic dialectic claimed that external causes become operative through internal causes. It is evident that internal factors plays a more important role in determining people's behavior.It is meaningless to weigh which factor poses more affections on people's behavior in modern life, Because in reality people are always dealing with the balance between external and internal ones. Sometimes you have to give up motivations pop up in your mind not letting them influence your behavior due to extremely external environment. But in most cases, we surely are capable of making our own decisions to commit a certain behavior. Therefore, it is fair to claim that both of the two sides have them power, but the internal outweigh the external.