a decent respect to the opinions of mankind requires that they should declare the causes
Out of respect for the opinions of mankind, the Founders will state the reasons that they are declaring independence from England and founding a new nation. These reasons, as we will learn in the following sentence, begin with the fundamental principles of government. Reasons are important to explaining America in a way they are not to other nations, because America is founded on the basis of ideas rather than ancestral traditions or divine revelation.
But what does it mean that the Founders respect mankind's opinions? It does not mean that they are drawing on those opinions. After all, the Declaration of Independence is a revolutionary document; it represents something brand new. Rather, this respect means that the Founders believe that any person of good will and common sense will be reasonable enough to understand the justice of the American's cause. In his letter to Henry Lee of May 8, 1823, Jefferson said that the Declaration was intended to "place before mankind the common sense of the subject, in terms so plain and firm as to command their assent, and to justify ourselves in the independent stand we are compelled to take."
James Madison wrote in Federalist no. 14: "Is it not the glory of the people of America, that whilst they have paid a decent regard to the opinions of former times and other nations, they have not suffered a blind veneration for antiquity, for custom, or for names, to overrule the suggestions of their own good sense, the knowledge of their own situation, and the lessons of their own experience? To this manly spirit, posterity will be indebted for the possession, and the world for the example of the numerous innovations displayed on the American theatre, in favor of private rights and public happiness."