Our panel of 91 professional philosophers has responded to

153
 questions about 
Sex
243
 questions about 
Justice
391
 questions about 
Religion
24
 questions about 
Suicide
284
 questions about 
Language
36
 questions about 
Literature
23
 questions about 
History
43
 questions about 
Color
88
 questions about 
Physics
77
 questions about 
Emotion
167
 questions about 
Freedom
208
 questions about 
Science
282
 questions about 
Knowledge
75
 questions about 
Beauty
4
 questions about 
Economics
124
 questions about 
Profession
27
 questions about 
Gender
81
 questions about 
Identity
5
 questions about 
Euthanasia
104
 questions about 
Art
38
 questions about 
Race
110
 questions about 
Biology
31
 questions about 
Space
75
 questions about 
Perception
364
 questions about 
Logic
58
 questions about 
Abortion
2
 questions about 
Culture
107
 questions about 
Animals
79
 questions about 
Death
1268
 questions about 
Ethics
54
 questions about 
Medicine
87
 questions about 
Law
282
 questions about 
Mind
34
 questions about 
Music
51
 questions about 
War
133
 questions about 
Love
151
 questions about 
Existence
68
 questions about 
Happiness
217
 questions about 
Education
96
 questions about 
Time
116
 questions about 
Children
58
 questions about 
Punishment
32
 questions about 
Sport
67
 questions about 
Truth
220
 questions about 
Value
571
 questions about 
Philosophy
67
 questions about 
Feminism
2
 questions about 
Action
69
 questions about 
Business

Question of the Day

Suppose S is the set of all things that are blue or green. Then my mug is in S because it's green and therefore satisfies "x is blue or x is green," and my pen is in the set S because it's blue and therefore satisfies "x is blue or x is green." Now it's true: satisfying "x is blue or x is green" picks out only one set: the set of all things that satisfy "x is blue or x is green." But the condition "x is green" is a different condition, and so is "x is blue."

However: when you say "being blue or green cannot be the reason why any other object is in any other set," there's an ambiguity. That could be read as "being blue cannot be the reason why an object is in any other set and being green cannot be the reason why an object is in any other set." In that case, however, it's false. Being green, and hence satisfying "x is green" puts my mug in the set G of all green things, and in the set S of all things that are either green or blue—that satisfy "x is green or x is blue." These two sets are not the same. One is a proper subset of the other. Being in the set S doesn't entail being in the set G, and also doesn't entail being in the set B, though it does entail being in either G or B.

The key is to formulate the membership condition so that there's no room for ambiguity. We have

o is in S if and only if o satisfies "x is green or x is blue."
o is in G if and only if o satisfies "x is green"
o is in B if and only if o satisfies "x is blue"

These are three different conditions that pick out three different sets. G and B are disjoint from one another and are proper subsets of S. The union of G and B is S. But the formulation "x is green or blue," while not wrong, masks the fact that "x is green or blue" amounts to "x is green or x is blue," and so is the disjunction (the "or") of two conditions. An object is in S if it satisfies either of those conditions. It's in G only if it satisfies the first, and in B only if it satisfies the second. But clearly by virtue of satisfying one condition ("x is green") my mug can be in S and also in G.