From The Roots of Romanticism (1967 lectures) by Isaiah Berlin
The First Attack on the Enlightenment
Three principles upon which the whole tradition of Western thought rests:
  1. All genuine questions can be answered.
  2. All answers are knowable.
  3. All answers are compatible with one another.

1. All genuine questions can be answered.

-  If the question has no answer, it is not a genuine question.
- We might not know the answer, but it does exist among an intellectual elite
- We might not know the answer, but it does exist in the next world, if not this one
- We might not know the answer, but it does exist in some Golden Age before the Fall and the Flood
- We might not know the answer, but it does exist in the future where we will arrive at the truth
- We might not know the answer, but it does exist, if not to men, then to God

2. All answers are knowable.

- Answers can be discovered by methods which can be taught (eventually) to others

3. All answers are compatible with one another.

- otherwise chaos results.

Hurrah! We're on the Path to Utopia!

If all answers to all questions can be put in the form of reasonable propositions and if all true propositions are discoverable, then there must be a description of an ideal universe- a Utopia.
The Enlightenment philosophes suggest that these answers were not to be obtained in traditional ways (through revelation, dogma or privilege). These answers can only be obtained through the scientific method (deductive in Mathematics and inductive in the natural sciences). There is no reason why these methods, which achieved astonishing breakthroughs in the worlds of physics and chemistry, can not be applied to politics, ethics, and aesthetics. What Newton had achieved in physics, we can achieve in society if we only dispel irrational habits and address our problems with reason.
The problem we face is putting the pieces of a huge jigsaw puzzle together, and in the end when all the pieces are fitted, we will have achieved a Utopia. It can be done if we dispel the irrational forces of superstition, ignorance and tyranny.

All We Have To Do is Solve The Problem of Constructing an Ideal Society!

How should we live?
What form of government should we choose?
Should we obey elites of specialists or should every man have the right to his own particular opinion?
Should the majority rule?
Is a republic preferable to a monarchy?          
Is it right for us to seek pleasure, or should we deny ourselves and focus on our duty?
Should one be an ascetic or a voluptuary?
Is 'the good' objective (something eternal and true for all men in all situations), or is it subjective (something which a particular person in a particular situation happened to like)?
Furthermore, certain universal ideals may be incompatible with each other:
Virtue consists in knowledge (and truth cannot make us miserable.)
All virtues are compatible with one another (freedom is compatible with equality; justice is compatible with mercy).

Uh-Oh...Cross-Currents in Enlightenment Thought
All the philosophes basically agreed on the notion that the principles upon which a virtuous society would be based could be obtained by scientific methods. However, they had contrasting opinions on many fundamental issues:
Human Nature:

Voltaire believed that man is hopelessly jealous, envious, wicked, corrupt and weak. In the state of nature we possess bestial instincts, and our environment reinforces these instincts rather than countering them through education. Therefore, society needs a government that will exert the most strenuous possible discipline on individuals; otherwise, we will probably be unable to cope with life at all.
Locke believed that human character is a malleable substance, a clay which competent educators and legislator can mold into a perfectly proper, rational state.
Rousseau believed that man is basically good (neither wicked, nor neutral) and has been ruined by institutions of his own making.

The Best Government?

Is a ruling elite necessary? (The mob will never learn; we are born with an inequality of gifts; unless this natural elite rules, we are bound for catastrophe.)

Voltaire believed that what men really want is happiness, contentment and peace. This can be achieved by living simply in a garden, pared and pruned, and deferring decisions to a wise philosophe who can teach us to be knowledgeable about the essential schools of knowledge: physics, chemistry, and mathematics.

Locke believed that in matters of ethics and politics, each man is his own expert. (We are all capable of determining right from wrong, and we have run into trouble when we have been misled by corrupt aristocrats, clergy, and kings.)

Rousseau believed that we must act collectively:
(Rather than trusting to an intellectual elite, we should act as a group, as one.)