English Question (philosophy)

Answer the following in one essay:

1) What is the relationship between impressions and ideas? 2) How are impressions and ideas different? 3) What are the principles of association ? (max. 400 words)

Please answer each question in 1 paragraph, so there will be 3 paragraphs for 3 questions. 

References :

1. http://www.gutenberg.org/files/9662/9662-h/9662-h.htm (Section 2 & 3)

2. http://plato.stanford.edu/entries/hume/   (Sections 4.1 & 4.3)