diff --git a/docs/command-definition.md b/docs/command-definition.md
index 027e9a8..1cd900a 100644
--- a/docs/command-definition.md
+++ b/docs/command-definition.md
@@ -37,11 +37,12 @@ Options are always optional (duh). If an option is required, then it should be a
 ## Default values
 
 ```php
-$app->command('greet [firstname] [lastname]', function () {
+$app->command('greet [firstname] [lastname] [--age=]', function () {
     // ...
 })->defaults([
     'firstname' => 'John',
     'lastname'  => 'Doe',
+    'age' => 25,
 ]);
 ```
 
diff --git a/src/Application.php b/src/Application.php
index 570b3c8..cee8a4a 100644
--- a/src/Application.php
+++ b/src/Application.php
@@ -10,6 +10,7 @@
 use Invoker\ParameterResolver\Container\ParameterNameContainerResolver;
 use Invoker\ParameterResolver\Container\TypeHintContainerResolver;
 use Invoker\ParameterResolver\ResolverChain;
+use ReflectionFunction;
 use Silly\Command\Command;
 use Silly\Command\ExpressionParser;
 use Symfony\Component\Console\Application as SymfonyApplication;
@@ -90,6 +91,10 @@ public function command($expression, $callable, array $aliases = [])
         $command = $this->createCommand($expression, $commandFunction);
         $command->setAliases($aliases);
 
+        if (is_callable($callable)) {
+            $command->defaults($this->defaultsViaReflection($callable));
+        }
+
         $this->add($command);
 
         return $command;
@@ -195,4 +200,21 @@ private function createCommand($expression, callable $callable)
 
         return $command;
     }
+
+    private function defaultsViaReflection(callable $callable)
+    {
+        $function = new ReflectionFunction($callable);
+
+        $defaults = [];
+
+        foreach ($function->getParameters() as $parameter) {
+            if (! $parameter->isDefaultValueAvailable()) {
+                continue;
+            }
+
+            $defaults[$parameter->name] = $parameter->getDefaultValue();
+        }
+
+        return $defaults;
+    }
 }
diff --git a/src/Command/Command.php b/src/Command/Command.php
index ca55cf7..60ceb75 100644
--- a/src/Command/Command.php
+++ b/src/Command/Command.php
@@ -39,19 +39,28 @@ public function descriptions($description, array $argumentAndOptionDescriptions
     /**
      * Define default values for the arguments of the command.
      *
-     * @param array $argumentDefaults Default argument values.
+     * @param array $defaults Default argument values.
      *
      * @return $this
      *
      * @api
      */
-    public function defaults(array $argumentDefaults = [])
+    public function defaults(array $defaults = [])
     {
         $definition = $this->getDefinition();
 
-        foreach ($argumentDefaults as $name => $default) {
-            $argument = $definition->getArgument($name);
-            $argument->setDefault($default);
+        foreach ($defaults as $name => $default) {
+            if ($definition->hasArgument($name)) {
+                $input = $definition->getArgument($name);
+            } elseif ($definition->hasOption($name)) {
+                $input = $definition->getOption($name);
+            } else {
+                throw new \InvalidArgumentException(
+                    "Unable to set default for [{$name}]. It does not exist as an argument or option."
+                );
+            }
+
+            $input->setDefault($default);
         }
 
         return $this;
diff --git a/tests/Command/CommandTest.php b/tests/Command/CommandTest.php
index 6599aa0..ec8de65 100644
--- a/tests/Command/CommandTest.php
+++ b/tests/Command/CommandTest.php
@@ -23,7 +23,7 @@ public function setUp()
         $this->application->setAutoExit(false);
         $this->application->setCatchExceptions(false);
 
-        $this->command = $this->application->command('greet [name] [--yell]', function () {});
+        $this->command = $this->application->command('greet [name] [--yell] [--times=]', function () {});
     }
 
     /**
@@ -34,6 +34,7 @@ public function allows_to_define_descriptions()
         $this->command->descriptions('Greet someone', [
             'name'   => 'Who?',
             '--yell' => 'Yell?',
+            '--times' => '# of times to greet?',
         ]);
 
         $definition = $this->command->getDefinition();
@@ -41,6 +42,7 @@ public function allows_to_define_descriptions()
         $this->assertEquals('Greet someone', $this->command->getDescription());
         $this->assertEquals('Who?', $definition->getArgument('name')->getDescription());
         $this->assertEquals('Yell?', $definition->getOption('yell')->getDescription());
+        $this->assertEquals('# of times to greet?', $definition->getOption('times')->getDescription());
     }
 
     /**
@@ -50,10 +52,51 @@ public function allows_to_define_default_values()
     {
         $this->command->defaults([
             'name' => 'John',
+            'times' => '1',
         ]);
 
         $definition = $this->command->getDefinition();
 
         $this->assertEquals('John', $definition->getArgument('name')->getDefault());
+        $this->assertEquals('1', $definition->getOption('times')->getDefault());
+    }
+
+    /**
+     * @test
+     */
+    public function allows_default_values_to_be_inferred_from_closure_parameters()
+    {
+        $command = $this->application->command('greet [name] [--yell] [--times=]', function ($times = 15) {
+            //
+        });
+
+        $definition = $command->getDefinition();
+
+        $this->assertEquals(15, $definition->getOption("times")->getDefault());
+    }
+
+    /**
+     * @test
+     */
+    public function setting_defaults_falls_back_to_options_when_no_argument_exists()
+    {
+        $this->command->defaults([
+            'times' => '5',
+        ]);
+
+        $definition = $this->command->getDefinition();
+
+        $this->assertEquals(5, $definition->getOption("times")->getDefault());
+    }
+
+    /**
+     * @test
+     * @expectedException \InvalidArgumentException
+     */
+    public function setting_unknown_defaults_throws_an_exception()
+    {
+        $this->command->defaults([
+            'doesnotexist' => '0',
+        ]);
     }
 }