Part A
Answer: The speaker thinks his neighbor is misguided in his views of the purpose of walls and fences.
Part B
Answer: The speaker believes that it is important to know what is being walled in or walled out. His neighbor believes that, regardless of the reason, good fences make good neighbors.
These responses highlight the differing perspectives of the speaker and his neighbor regarding the significance and purpose of the wall, thereby contributing to the central idea of the poem.