diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3d657d8..e8e27a5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 #### Additions
 * Refactor to support class variables as well
+* Use declared subject as default target for with helper
 
 ## 0.1.0
 
diff --git a/README.md b/README.md
index e31baf1..36fd0fa 100644
--- a/README.md
+++ b/README.md
@@ -61,6 +61,8 @@ end
 config.instance_variable_get('@version')     # => 1
 ```
 
+:warning: The target `on` is set explicitly in this case. If you omit this argument, `self` will be used as target by default.
+
 Class variables can be substituted as well:
 
 ```ruby
@@ -137,6 +139,8 @@ with '@version', on: Config do
 end
 ```
 
+:warning: The target `on` is set explicitly in this case. If you omit this argument, `:subject` will be used as target by default which refers to the subject defined by the `subject {}` helper.
+
 It's safe to use multiple `with` statements within one `describe` block.
 
 (The spec integration is borrowed from [minitest-around](https://rubygems.org/gems/minitest-around) for elegance and compatibility.)
diff --git a/lib/minitest/substitute/spec.rb b/lib/minitest/substitute/spec.rb
index 52f2e45..2ec62cb 100644
--- a/lib/minitest/substitute/spec.rb
+++ b/lib/minitest/substitute/spec.rb
@@ -8,12 +8,14 @@
   #
   # @param variable [String] instance or global variable name
   # @param substitute [Object] temporary substitution value
-  # @param on [Object, nil] substitute in the context of this object
+  # @param on [Object, Symbol, nil] substitute in the context of this object
+  #   or in the context of the declared subject if +:subject+ is set
   # @yield temporary substitution value (takes precedence over +substitute+ param)
-  def with(variable, substitute=nil, on: self)
+  def with(variable, substitute=nil, on: :subject)
     substitute = yield if block_given?
     substitutor = Minitest::Substitute::Substitutor.new(variable, substitute, on: on)
     before do
+      substitutor.on = subject if on == :subject && respond_to?(:subject)
       substitutor.commit
     end
     after do
diff --git a/lib/minitest/substitute/substitutor.rb b/lib/minitest/substitute/substitutor.rb
index 99087f3..bd3397a 100644
--- a/lib/minitest/substitute/substitutor.rb
+++ b/lib/minitest/substitute/substitutor.rb
@@ -6,12 +6,14 @@ class Substitutor
 
       EVAL_METHODS = [:instance_eval, :instance_eval, :class_eval].freeze
 
+      attr_writer :on
+
       def initialize(variable, substitute, on:)
         @variable, @substitute, @on = variable, substitute, on
-        @original = get
       end
 
       def commit
+        @original = get
         set @substitute
       end
 
diff --git a/spec/factory.rb b/spec/factory.rb
index f398f9f..e956c71 100644
--- a/spec/factory.rb
+++ b/spec/factory.rb
@@ -9,5 +9,4 @@ def initialize
   @@counter = 0
 end
 
-$_spec_config_instance = Config.new
-$_spec_global_variable = :original
+$spec_global_variable = :original
diff --git a/spec/lib/minitest/substitute/spec_spec.rb b/spec/lib/minitest/substitute/spec_spec.rb
index 15dd5a0..9dc6a04 100644
--- a/spec/lib/minitest/substitute/spec_spec.rb
+++ b/spec/lib/minitest/substitute/spec_spec.rb
@@ -6,17 +6,21 @@
 describe :with do
   10.times do   # test rollback even though order is random
     describe "instance variable" do
+      subject do
+        Config.new
+      end
+
       context 'untouched' do
         it "returns the original value" do
-          _($_spec_config_instance.instance_variable_get(:@version)).must_equal 1
+          _(subject.instance_variable_get(:@version)).must_equal 1
         end
       end
 
       context 'substituted' do
-        with '@version', 2, on: $_spec_config_instance
+        with '@version', 2
 
         it "returns the substitute value" do
-          _($_spec_config_instance.instance_variable_get(:@version)).must_equal 2
+          _(subject.instance_variable_get(:@version)).must_equal 2
         end
       end
     end
@@ -24,35 +28,39 @@
 
   10.times do   # test rollback even though order is random
     describe "class variable" do
+      subject do
+        Config
+      end
+
       context 'untouched' do
         it "returns the original value" do
-          _(Config.class_variable_get(:@@counter)).must_equal 0
+          _(subject.class_variable_get(:@@counter)).must_equal 0
         end
       end
 
       context 'substituted' do
-        with '@@counter', 42, on: Config
+        with '@@counter', 42
 
         it "returns the substitute value" do
-          _(Config.class_variable_get(:@@counter)).must_equal 42
+          _(subject.class_variable_get(:@@counter)).must_equal 42
         end
       end
     end
   end
 
-  10.times do   # test rollback even though order is random
+10.times do   # test rollback even though order is random
     describe "global variable" do
       context 'untouched' do
         it "returns the original value" do
-          _($_spec_global_variable).must_equal :original
+          _($spec_global_variable).must_equal :original
         end
       end
 
       context 'substituted' do
-        with "$_spec_global_variable", 'oggy'
+        with "$spec_global_variable", 'oggy'
 
         it "returns the substitute value" do
-          _($_spec_global_variable).must_equal 'oggy'
+          _($spec_global_variable).must_equal 'oggy'
         end
       end
     end
@@ -78,22 +86,26 @@
 
   10.times do   # test rollback even though order is random
     describe "multiple substitutions" do
+      subject do
+        Config.new
+      end
+
       context 'untouched' do
         it "returns the original values" do
-          _($_spec_config_instance.instance_variable_get(:@version)).must_equal 1
-          _($_spec_config_instance.instance_variable_get(:@released_on)).must_equal '2023-08-24'
+          _(subject.instance_variable_get(:@version)).must_equal 1
+          _(subject.instance_variable_get(:@released_on)).must_equal '2023-08-24'
         end
       end
 
       context 'substituted' do
-        with '@version', 2, on: $_spec_config_instance
-        with '@released_on', on: $_spec_config_instance do
+        with '@version', 2
+        with '@released_on' do
           '2012-12-12'
         end
 
         it "returns the substitute values" do
-          _($_spec_config_instance.instance_variable_get(:@version)).must_equal 2
-          _($_spec_config_instance.instance_variable_get(:@released_on)).must_equal '2012-12-12'
+          _(subject.instance_variable_get(:@version)).must_equal 2
+          _(subject.instance_variable_get(:@released_on)).must_equal '2012-12-12'
         end
       end
     end
diff --git a/spec/lib/minitest/substitute/with_spec.rb b/spec/lib/minitest/substitute/with_spec.rb
index ee0f1cf..1688a7b 100644
--- a/spec/lib/minitest/substitute/with_spec.rb
+++ b/spec/lib/minitest/substitute/with_spec.rb
@@ -6,11 +6,12 @@
 describe :with do
   describe "instance variable" do
     it "substitutes the value for the duration of a block" do
-      _($_spec_config_instance.instance_variable_get(:@version)).must_equal 1
-      with '@version', 2, on: $_spec_config_instance do
-        _($_spec_config_instance.instance_variable_get(:@version)).must_equal 2
+      config = Config.new
+      _(config.instance_variable_get(:@version)).must_equal 1
+      with '@version', 2, on: config do
+        _(config.instance_variable_get(:@version)).must_equal 2
       end
-      _($_spec_config_instance.instance_variable_get(:@version)).must_equal 1
+      _(config.instance_variable_get(:@version)).must_equal 1
     end
   end
 
@@ -26,11 +27,11 @@
 
   describe "global variable" do
     it "substitutes the value for the duration of a block" do
-      _($_spec_global_variable).must_equal :original
-      with "$_spec_global_variable", 'oggy' do
-        _($_spec_global_variable).must_equal 'oggy'
+      _($spec_global_variable).must_equal :original
+      with "$spec_global_variable", 'oggy' do
+        _($spec_global_variable).must_equal 'oggy'
       end
-      _($_spec_global_variable).must_equal :original
+      _($spec_global_variable).must_equal :original
     end
   end
 
@@ -46,10 +47,11 @@
 
   describe "nested substitutions" do
     it "substitutes the values for the duration of a block" do
-      with '@version', 2, on: $_spec_config_instance do
-        with '@released_on', '2012-12-12', on: $_spec_config_instance do
-          _($_spec_config_instance.instance_variable_get(:@version)).must_equal 2
-          _($_spec_config_instance.instance_variable_get(:@released_on)).must_equal '2012-12-12'
+      config = Config.new
+      with '@version', 2, on: config do
+        with '@released_on', '2012-12-12', on: config do
+          _(config.instance_variable_get(:@version)).must_equal 2
+          _(config.instance_variable_get(:@released_on)).must_equal '2012-12-12'
         end
       end
     end