diff --git a/lib/Twig/BaseNodeVisitor.php b/lib/Twig/BaseNodeVisitor.php
index 8c42cfb25..c2f66c3b8 100644
--- a/lib/Twig/BaseNodeVisitor.php
+++ b/lib/Twig/BaseNodeVisitor.php
@@ -9,8 +9,8 @@
  * file that was distributed with this source code.
  */
 
-require_once __DIR__.'/Environment.php';
-require_once __DIR__.'/Node.php';
+class_exists('Twig_Environment');
+class_exists('Twig_Node');
 
 /**
  * Twig_BaseNodeVisitor can be used to make node visitors compatible with Twig 1.x and 2.x.
diff --git a/lib/Twig/Compiler.php b/lib/Twig/Compiler.php
index ca4789ea7..a61ae6ffe 100644
--- a/lib/Twig/Compiler.php
+++ b/lib/Twig/Compiler.php
@@ -10,7 +10,7 @@
  * file that was distributed with this source code.
  */
 
-require_once __DIR__.'/Node.php';
+class_exists('Twig_Node');
 
 /**
  * Compiles a node to PHP code.
diff --git a/lib/Twig/Error.php b/lib/Twig/Error.php
index d59edb9e1..7d0d492ab 100644
--- a/lib/Twig/Error.php
+++ b/lib/Twig/Error.php
@@ -9,7 +9,7 @@
  * file that was distributed with this source code.
  */
 
-require_once __DIR__.'/Source.php';
+class_exists('Twig_Source');
 
 /**
  * Twig base exception.
diff --git a/lib/Twig/Extension.php b/lib/Twig/Extension.php
index 54fb3866b..1d62bab30 100644
--- a/lib/Twig/Extension.php
+++ b/lib/Twig/Extension.php
@@ -9,7 +9,7 @@
  * file that was distributed with this source code.
  */
 
-require_once __DIR__.'/Environment.php';
+class_exists('Twig_Environment');
 
 abstract class Twig_Extension implements Twig_ExtensionInterface
 {
diff --git a/lib/Twig/Extension/Profiler.php b/lib/Twig/Extension/Profiler.php
index 4163b2406..723a63e4f 100644
--- a/lib/Twig/Extension/Profiler.php
+++ b/lib/Twig/Extension/Profiler.php
@@ -9,7 +9,7 @@
  * file that was distributed with this source code.
  */
 
-require_once __DIR__.'/../Profiler/Profile.php';
+class_exists('Twig_Profiler_Profile');
 
 class Twig_Extension_Profiler extends Twig_Extension
 {
diff --git a/lib/Twig/ExtensionInterface.php b/lib/Twig/ExtensionInterface.php
index 6e091adbc..81b68dbad 100644
--- a/lib/Twig/ExtensionInterface.php
+++ b/lib/Twig/ExtensionInterface.php
@@ -9,7 +9,7 @@
  * file that was distributed with this source code.
  */
 
-require_once __DIR__.'/Environment.php';
+class_exists('Twig_Environment');
 
 /**
  * Interface implemented by extension classes.
diff --git a/lib/Twig/Node.php b/lib/Twig/Node.php
index aaaa58f76..5ad374748 100644
--- a/lib/Twig/Node.php
+++ b/lib/Twig/Node.php
@@ -10,7 +10,7 @@
  * file that was distributed with this source code.
  */
 
-require_once __DIR__.'/Compiler.php';
+class_exists('Twig_Compiler');
 
 /**
  * Represents a node in the AST.
diff --git a/lib/Twig/NodeVisitorInterface.php b/lib/Twig/NodeVisitorInterface.php
index 1b326403a..36e81efb2 100644
--- a/lib/Twig/NodeVisitorInterface.php
+++ b/lib/Twig/NodeVisitorInterface.php
@@ -9,8 +9,8 @@
  * file that was distributed with this source code.
  */
 
-require_once __DIR__.'/Environment.php';
-require_once __DIR__.'/Node.php';
+class_exists('Twig_Environment');
+class_exists('Twig_Node');
 
 /**
  * Twig_NodeVisitorInterface is the interface the all node visitor classes must implement.
diff --git a/lib/Twig/Parser.php b/lib/Twig/Parser.php
index b01ea0c78..3d491a28c 100644
--- a/lib/Twig/Parser.php
+++ b/lib/Twig/Parser.php
@@ -10,8 +10,8 @@
  * file that was distributed with this source code.
  */
 
-require_once __DIR__.'/Node.php';
-require_once __DIR__.'/TokenStream.php';
+class_exists('Twig_Node');
+class_exists('Twig_TokenStream');
 
 /**
  * Default parser implementation.
diff --git a/lib/Twig/Test/NodeTestCase.php b/lib/Twig/Test/NodeTestCase.php
index 89b2ac842..ff26c92cd 100644
--- a/lib/Twig/Test/NodeTestCase.php
+++ b/lib/Twig/Test/NodeTestCase.php
@@ -9,8 +9,8 @@
  * file that was distributed with this source code.
  */
 
-require_once __DIR__.'/../Environment.php';
-require_once __DIR__.'/../Node.php';
+class_exists('Twig_Environment');
+class_exists('Twig_Node');
 
 use PHPUnit\Framework\TestCase;
 
diff --git a/lib/Twig/TokenParserInterface.php b/lib/Twig/TokenParserInterface.php
index 5c32fa905..2114a34c6 100644
--- a/lib/Twig/TokenParserInterface.php
+++ b/lib/Twig/TokenParserInterface.php
@@ -9,8 +9,8 @@
  * file that was distributed with this source code.
  */
 
-require_once __DIR__.'/Parser.php';
-require_once __DIR__.'/Token.php';
+class_exists('Twig_Parser');
+class_exists('Twig_Token');
 
 /**
  * Interface implemented by token parsers.